Nearly 1 million military and federal retirees -- including many of the 100,000 living here -- stand to lose half their future pension increases for the next three years if Congress approves a pending package of spending cuts.

Most retirees who would be affected by the changes are former military personnel. But the proposal, due to be voted on this week, would also affect 1 in every 10 civilian federal retirees.

Under the plan, retirees who have not reached their 62nd birthday by March 1, 1983, would get only one-half the COLA (cost-of-living adjustment) that older retirees would get in 1983 to help keep pace with inflation. The same cutoff would apply in each of the next two years.

The package, which also contains another unpleasant surprise for military retirees, is part of a $30 billion, three-year spending cut worked up by Senate-House conferees. If approved by the full Congress, the pension changes would save an estimated $4 billion by shaving the next three COLAs for many retirees. This is what would happen:

Full COLA raises in 1983, 1984 and 1985 would be limited to retirees who are 62 or older. Retirees who are under 62 would get only 50 percent (one-half) of the adjustment, which is based on the rise in living costs for the previous year as measured by the Consumer Price Index.

To be eligible for a full COLA raise in any of those years, retirees would have to be 62 or older before March 1 of that particular year.

Federal-military retirees received an 8.7 percent COLA raise in March. If the proposed changes had been in effect then, retirees under age 62 would have received less than 4.4 percent.

* The COLA adjustment now set to go into effect next March (and payable in April retirement checks) would be delayed until April 1, 1983. The COLA increase would show up in annuity checks sent out for early May delivery.

* 1984's COLA adjustment would not be effective until May of that year, payable in June checks.

* The 1985 COLA adjustment would not go into effect until June of that year. It would show up in annuity checks mailed for July delivery.

* Military retirees who are working for the government as civilians (getting a military pension check and a federal paycheck) would take a salary reduction equal to the amount of any COLA raise they got. In other words, the retiree could get a 5 percent pension increase, but then his federal paycheck would be reduced by the same amount.

The reduced COLA raise for persons under age 62 would not apply to individuals (widows and widowers of U.S. retirees) getting survivor benefits or those retired on disability.

Officials estimate that the age- 62 COLA cutoff would affect 175,000 civilian retirees, and because they must retire earlier, more than 800,000 military retirees.

Under-62 government retirees, and persons about to retire, are furious over the proposed change, which they see as blatant age discrimination against younger retirees.

Senate-House conferees voted for the provision in lieu of an earlier proposal that would have limited COLA raises to 4 percent for all retirees for the next three years.

Senate-House conferees, who were under the gun to cut $1 billion more than they actually did from federal/military pension costs, decided it would be better to make younger retirees take half a COLA raise rather than limit everybody to a maximum 4 percent inflation adjustment.

Their logic, whether you like it or not, is that older retirees usually get smaller annuities and are less likely to take second jobs after leaving government than the younger, under-62 group.