A popular trivia question for Bullet fans when the National Basketball Association season opens in the fall could well be: What do Kevin Grevey, Elvin Hayes, Wes Unseld and Phil Chenier have in common?

Answer: They are the only players who were here two seasons ago.

A two-thirds turnover in the 12-man roster in two seasons is a possibility, team sources indicate, in view of the college player draft Friday and the probably loss of several veteran players.

The Bullets picked 6-foot-7 Greg Ballard of Oregon and 6-9 Maurice (Bo) Ellis of Marquette in the first round of the draft. Unless they are used as trade bait, both figure to make next season's team.

Phil Walker, the Bullet's second-round pick, has an excellent shot at making the team, as does fourth-round draft choice David Reavis, a multitalented swing man, formerly of Mackin High School.

And Bob Dandridge, the free agent forward formerly of the Milwaukee Bucks, must be counted among next year's Bullets.

The Bullets haven't signed him, but he wants to play for Washington. The Bullets want him too and the word around the NBA is that it will be a surprise if the Bullets don't get him.

What compensation they would have to give up isn't certain, but Ellis ane next year's No. 1 draft choice would be acceptable offerings to the Bucks.

If the Bullets got Dandridge, they would figure to finish so high in the NBA standings that their No. 1 pick next year would come late in the first round.

If Ballard, Dandridge, Walker and Reavis are added, then four people must go. And as coach Dick Motta coincidentally put it, "There will be four competitive spots next year."

The Bullets had 13 players on the payroll last year, with Jimmy Jones on injured reserve. Of the active 12, Grever, Hayes, Unseld, Chenier, Tom Henderson, Larry Wright and Mitch Kupchak are sure to work back. Joe Pace will have to work harder if he wants to stay, and he propbably will.

Mike Riordan, Jones, Bob Weiss, Leonard Gray and Unseld are free agents.

Unseld is expected to sign soon and Weiss will retire. But Gary, Riordan and Jones are expendable and probably will not be back with the Bullets.

Gary injured his knee playing tenis in Seattle recently and general manager Bob Ferry said he doesn't know what the player's plans are. Ferry noted, however, that he was "not going to get into a bidding war over my own people."

Dave Bing, whom the Bullets have under contract for one more season, will probably not be asked back. He and Motta didn't see eye to eye after Bing's recent public criticism of the coach and it's hard to see a place for Bing on next year's team. Here is what the Bullet roster could look like come opening night in October:

Kupchak, Wright, Unseld, Chenier, Henderson, Dandridge, Ballard, Pace, Walker, Hayes, Grevey and Reavis.

Reavis is a real question mark - a dark horse on a tentative roster. Another draftee, Jerry Schellenberg of Wake Forest, is the same type of player and should battle Reavis for a spot. Motta may also choose to go with a fifht guard for that spot or try to pick up a veteran forward to fill out the roster.