Federal workers who want to get full lump-sum pension payments must have their retirement approved and effective no later than Jan. 2 to qualify under a surprise, last-minute change approved yesterday by White House- congressional conferees working on a compromise deficit reduction package.

Because of the Friday New Year's holiday, papers must be in -- and approved -- by Dec. 31.

Anyone whose annuity commences after the deadline and before Oct. 1, 1989, will be affected by new rules. They stretch the lump-sum payments over two years. The first payment, 60 percent of the lump sum, would come next year. The second, 40 percent, would be due in 1989.

The change in lump-sum payment rules may produce a crowd, starting today, at agency retirement offices.

Until the change is effective, any retiring employe can get a single lump-sum payment equal to the employe's pension plan contributions. For longtime, high-paid people, that can be $70,000 or more. For the typical retiree, the lump sum is about $20,000 before taxes.

In other conference action, thousands of federal workers, mostly women, planning to collect their federal pension plus a Social Security spouse or widow benefit get a break if they join the new retirement system by Dec. 31.

Here's a rundown of the two complex, important changes:

Pension Offset: Under the Public Pension Offset Law, anyone who gets a federal or public pension and who applies for an (unearned) spousal or widow benefit under Social Security has that benefit reduced sharply.

The White House- congressional compromise exempts anyone joining the new Federal Employees Retirement System by Dec. 31 from the offset that already applies to people under the old Civil Service Retirement System.

After Dec. 31, employes joining FERS must remain under the system for five years before they can escape the Social Security offset, which reduces the unearned Social Security benefit $2 for every $3 the individual gets in a federal or public pension.

This means that federal workers must join FERS by Dec. 31 to avoid the pension offset. If there is a retirement open season next year, anyone signing up for FERS will be subject to the offset provision.

Lump Sum: The lump-sum change will be important to thousands planning to retire by October 1989.

Under the temporary change, people whose annuity commences after the effective date of the change will get only 60 percent of their lump-sum payment next year and the remainder -- plus interest -- the following year.

That change, inserted at the last minute by conferees, will make it impossible for many people who are close to retirement eligibility, but not eligible to retire by Jan. 2, to beat the deadline and get a full lump-sum payment.

Warning: If agency retirement officers don't understand the new rules changes, bear with them. This column may be the first notice most of them have of the situation and the important new deadlines.

But to get in under the wire, run, don't walk, to get the papers in!