Yesterday's delicatessen is gone and replaced by a sushi restaurant, which give way to an instant-printing shop and then to an "adult-only" bookstore.

And, on the next trip back, not only is the adult bookstore gone but so is everything else in a square-block radius -- reduced to a yawning hole in the ground from which the footings for a new high-rise poke up like exposed nerve ends.

The features constantly are changing but the face remains the same. Or, as Manhattan's natives say: "It'll be a great city -- once they get it finished."

But, while the expensiveness of New York City's real estate has, in the past, always been the major determinant in the constant shifting of the skyline, new considerations are slowing the age-old New York developer's solution for all problems: Tear it down.

Soaring labor and material costs -- and sky-high interest rates on construction loans that make time the most expensive element of all -- have put a new, and for Manhattan developers, foreign emphasis on rehabilitation of old midtown structures. Even an extensive rehab of a single-family home, as anyone who has lived through the ordeal can testify, can be a tremendously involved operaton.

Completely gutting, without tearing down, a 20- or 30-story building with zero clearance between it and its neighbors is an architectural and construction challenge of mind-boggling dimensions.

And two mid-Manhattan projects just three blocks apart -- one completed just eight months ago and the other scheduled to open next month -- underline other the diversity and the creativeness of the new wave of rehabilitation sweeping across this city of eternal change.

On vibrant 42d Street between Lexington and Grand Central Station the old Commodore Hotel, for years, had been going quietly to seed like a once-stately burlesque queen going to flab. Frayed carpeting and rips in the upholstered sofas during the lobby were a far cry from the grandeur of the hotel that was circus king John Ringling's home-away-from-home, Gen. Dwight D. Eisenhower's headquarters during his presidential campaign and the scene of the triumphant welcome home banquet for Charles A. Lindbergh.

By the mid-1970s, the Commodore -- named for Cornelius Vanderbilt and opened to the public in 1916 -- had become the stopping place of last resort for weary commercial travelers. It had become almost possible for the doormen to keep 42d Street's hookers and winos out of the lobby, and the massage parlor on the mezzanine was a source of embarrassment for out-of-towners still checking into the now-gloomy hotel out of a sense of loyalty.

In 1976, with a whimper, the Commodore closed its doors, a year after Manhattan real estate developer Donald Trump it for $10 million.

What he ahd in mind was a joint venture with the Hyatt Hotels Corp. that, before the completion of the massive rehabilitation project last September was to see still another $100 million poured into it. Thus was born the 30-story Grand Hyatt.

For the 23-year-old Hyatt organization, the Commodore project was the most ambitious undertaking to date for a company that had started out concentrating on airport hotels -- the first, in 1957, being the Airport Hyatt House at Los Angeles International Airport, but followed quickly by similar installations at, or near, the airports at San Francisco, Seattle, San Jose and Chicago.

But it was as recently as 1967 that Hyatt began cutting its swath -- with the Hyatt Regency Atlanta -- by breaking dramatically with traditional hotel design through the creation of atrium lobbies complete with trees and hanging vines, spectacular sculptures, glass elevators and revolving rooftop restaurants. All of which have almost become a Hyatt trademark.

In its decision to gut the Commodore, however, an already complicated chore was further complicated by Trump/Hyatt's desire to retain, but completely redo the old hotel's magnificent 20,000-square-foot banquet hall -- the third largest in the city and capable of accomodating 2,000 diners. Also, since the Empire State Ballroom sat directly over the lobby, Hyatt's by-now traditional, multi-floored atrium design had to be modified -- but still spectacularly enough to retain the theme of openness, broad expanses of glass and a fountain topped by a 100-foot-high brass sculpture by Peter Lobello.

And, continuing the openness at the second level of the lobby, is the "Sun-Garden," a plant-filled greenhouse cantilevered out over 42d Street and offering a panoramic view of that lively throughfare.

Any temptation to retain, but completely refurbish, the guest rooms ran into a 64-year change in tastes: Virtually all of the 1,900 rooms of the old Commodore were far too small for the 1980 traveler. The total number of rooms was reduced to 1,407 and 125 suites were created.

Just three blocks away, between Park and Lexington and extending form 45th to 46th streets, the rehabilitation project slated to open next month confronted a different set of challenges: not to retain any aspects of old glamor, as in the case of the Commodore-turned-Grand Hyatt, but to build upon and obliterate all traces of a classic turn-of-the-century example of New York Ugly -- the Railroad Mail Service Building.

In 1914, a four-story, fortress-like quadrangle was built around an air shaft to provide cross ventilation to as many offices as possible in those pre-air-conditioning days. In 1920, 13 additional stories were added to it, but did nothing to soften the fortress appearance.

When the building was acquired by the Toronto-based Olympia and York Developments in the late 1970s, the decision to rehabilitate rather than tear it down was, again, a simple matter of economics.

Nathaniel Stein, senior vice president of the Toronto firm, said that one problem "was the fact the building sits on a complex network of railroad tracks which had to be kept operative. And the over consideration was that the fourth floor was occupied by the Metropolitan Trust Authority and there wasn't any time to get them out until 1980."

But, by beginning the simultaneous demolishment and reenforcement of the upper stories while the fourth floor remained occupied and the trains continued running, "we saved about a year by being able to begin work in early 1980," Stein said.

There may have been a slight additional construction saving in rehabilitation rather than starting from scratch, Stein added, but the horrendous costs that would have been incurred in moving and re-routing the railroad tracks "were pretty well washed out by the additional reenforcing that was required as the structural steel and floor arches at the upper levels were exposed to wind pressures.

The big Canadian firm owns an estimated 50 million square feet of office space in Canada, Europe and United States, and is the second biggest commercial real estate holder (next to Rockefeller) in New York City.

Long noted for the distinctiveness of its high-rises, such as the ITT Building and the American Brands Building in mid-Manhattan, Olympia and York was determined that the Park/Lexington project, called the Park Avenue Atrium, was going to be "a first" for jaded New Yorkers.

Designed by Edward Durell Stone, with Emery Roth & Sons as associate, the Atrium is a 23-story greenhouse, an interior park enclosed at the top by a glass roof. Most major office space will have natural light by virtue of fronting on either the street side of the building or on the atrium. Each floor is divided into four quadrants, each having its own controls for heating, air conditioning, lighty and ventilation.