It's all in a preliminary stage of decision-making, but State Department insiders say 1988 may bring a big shuffle of some of the most important U.S. diplomatic posts.

Current ideas on the yellow-lined pads of top officials would see Undersecretary for Management Ronald I. Spiers becoming ambassador to Canada. He would replace Thomas M. Niles, who would be reassigned from Ottawa to become director of the department's Bureau of Intelligence and Research.

The State Department intelligence chief, Morton I. Abramowitz, would become ambassador to India. This post is held by John Gunther Dean, who, like the others in "Operation Big Switch," is a career Foreign Service officer. Dean got his current post three years ago over the objections of the State Department hierarchy but with the strong backing of then-White House deputy chief of staff Michael K. Deaver.

Meanwhile, Ambassador to Israel Thomas R. Pickering is being penciled in to succeed Spiers in the top State management position. His successor in Tel Aviv would be William M. Brown, the ambassador to Thailand. To complete the circle, the new top diplomat in Bangkok would be Daniel A. O'Donohue, an Asia veteran who is deputy director of the State Department's policy planning staff.

In terms of personal rank, Spiers and Pickering are the two highest-ranking career officers in the Foreign Service.

Another important shift at the discussion stage is the replacement of U.S. ambassador to Japan Mike Mansfield, who is undergoing surgery in Hawaii. If the 84-year-old former Senate Democratic leader decides it's time to retire, the likely successor would be State's No. 3 official, Undersecretary for Political Affairs Michael H. Armacost, a Japan veteran.

If this shift takes place, it will require further shuffles to fill Armacost's shoes. FEC's New Leader . . .

With '88 around the corner, the Federal Election Commission is gearing up. Thomas J. Josefiak has been elected as chairman for the next calendar year, succeeding Scott E. Thomas. Named as vice chairman is Danny Lee McDonald. Josefiak is a Republican and McDonald a Democrat. Where's the Foam . . .

Eat at the Capitol in good conscience, the National Wildlife Federation says. Recently, the group's magazine reports, managers of the Hill cafeterias removed all foam cups and plates manufactured with chlorofluorocarbons. For the federation, that's another step, though symbolic, toward saving the atmosphere's ozone layer. Ground Zero . . .

This month, House-Senate negotiators agreed to a plan that is almost certain to place all the nation's nuclear waste in Nevada. Rep. Barbara F. Vucanovich (R-Nev.) had a few biting words about the deal. Congress, she said, "is behaving like a pack of wolves going in for the kill."