"The slums of the future are in the suburbs."

"City housing values will soar as the affluent middle class returns. It will be a disaster for the displaced poor."

"Power and prestige in the future will be determined by access to energy. The cities are energy-conserving, the suburbs energy-wasteful. The smart money, the smart homeowner, will gravitate to the center city."

"The poor always end up in the most undesirable areas. Many will be forced into the cheaply built subdivisions thrown up after World War II - the matchbox-like, poorly insulated housing the middle class will desert."

"You'll see suburbs trying to annex their center cities."

As far-fetched as such statements may seem, I have heard them seriously advanced by responsible city officials and urban experts in the past several weeks.

Despite continuing suburban business and home expansion, there's a sense of near jubilation among city spokesman who believe the modest flow of young professional people into the cities - mostly singles and childless couples - will soon turn into a tidal wave because of the energy crisis and changing lifestyles.

But "gentrification" - a term the English use to denote the return of the "gentry" to inner-city neighborhoods - is not a problem-free process. In such neighborhoods as Philadelphia's Queen Village, Boston's South End, Washington's Capitol Hill and Adams Morgan and Columbus, Ohio's German Village, the restoration process has been bad news for many blue-collar ethnic and minority residents.

Though some cities are so "hopeless" they may never revive, a National Urban Coalition survey of 43 cities, large and small, shows that displacement - the recycling of poor people at the convenience of rich people - is a widespread phenomenon across the country.

"It would be an ironic turn if the rich rediscovered the old city neighborhoods and pushed out the poor into suburban slums," says Sen. William Proxmire (D-Wis.).

No one believes gentrification should - or could - be stopped in its tracks. In its first stages, it's all to the good: Abandoned, "bombed-out" buildings are restored by "urban pioneers"; the newcomers give life to old businesses and stimulate creation of new ones, stage city festivals and help their less-affluent neighbors by fighting demolition-style urban renewal, redlining, and superhighways that rip neighborhoods apart.

But before long, property values begin to soar. Houses that sold before for $10,000 or $20,000 suddenly cost $50,000 to $70,000. Landlords begin to evict poor tenants, either selling out at substantial profit or renovating properties for the affluent returnees. Assessments rise rapidly, so that many poor or elderly residents can no longer afford the property taxes on homes they may have occupied for many years.

Gentrification begins to smell like block-busting in reverse: Street by street, neighborhood by neighborhood, one kind of homogeneous, single-income and single-culture community is totally replaced by another.

The tragedy of this, neighborhood spokesmen say, is not just for the displaced poor. It also means loss of a golden opportunity to create richly diverse neighborhoods, rather than the poor or rich, black or white ghettoes so typical of America now and in the past

A compelling case for diversity is made by Conrad Weiler, chairman of the Alliance for Neighborhood Government and president of Queen Village Neighbors in Philadelphia. Neighborhood diversity, he says, includes but goes beyond racial integration. It means mixing people of different incomes, educations, occupations and ethnic and religious backgrounds. It means ending the isolation of the old from the rest of society. It "can help fight crime by having many different eyes on the same streets all the time."

Diversity, Weiler continues, puts people closer to where they need to be for jobs, shopping or relatives. It emphasizes accessibility over mobility, less energy use and automobile dependency. It reduces panic selling and real-estate speculation. It means mixing single-family homes with apartments, light industry with commercial, the corner store with residences, "the artisan living upstairs and his workshop and store downstairs."

Diversity can also reduce or eliminate the need for school busing. The schools, imperiled by the policy of attracting only affluent young singles and childless married couples now advocated in such cities as Seattle, would more easily be "saved." Unless a city can attract families with children, says Portland's Mayor Neil Goldschmidt, "you'll end up with a school system that's still basically black and poor." (There's a danger, he notes, that neither the elderly, the poor nor the childless gentry will be inclined to vote for school taxes.)

No one has yet thought through a coherent strategy to disperse middle-class reinvestment across center cities, to protect the poor and promote diversity - indeed few people have considered the problem at all.

Hartford, however, is starting to mothball - rather than demolish - abandoned buildings, with the expectation of selling some off to affluent returnees and using the profits to rehabilitate others for the poor.

Washington requires absentee landlords to offer tenants of rent-controlled units rights of first refusal to match the purchase offer of a third party. The missing element is financing: federal housing-subsidy laws may have to be adapted to make it easier for tenants - not just developers - to finance rehabilitations.

A city can postpone increases in assessments until a property is sold, or permit only gradual assessment increases; it can also place a high tax on profits of speculators who don't actually live in buildings they buy and quickly sell.

Finally, broadly participative neighborhood organizations can be given major power - under city-set guidelines - to monitor new development coming into their communities, controlling its place and nature to protect existing residents and enhance diversity.

Intense conflict is likely at every step. After years of fiscal stringency and reduced services, for instance, many mayors may care little if the poor are pushed out - all the way to the suburbs.

If that happens, it willbe the shoddily built, energy-inefficient suburban housing tracts of the past 30 years - candidates for blockbusting and slumhood in the '80s and '90s.

The irony is that the suburbs, bastions of exclusion until now, may develop radically new attitudes. They'll favor diversified city neighborhoods, lest the poor move outward in intolerably vast numbers. Even more important, many will feel compelled to seek diversity for themselves as energy shortages become acute and the amenity-rich cities pose even stiffer competition.