Investment levels in the new federal thrift plan -- which allows employes to make tax-deferred investments of up to 15 percent of salary -- would be protected from future cutbacks if Congress goes along with proposals to exempt the plan from rules that apply to similar investment programs in the private sector.

Most federal workers can now invest from 5 to 10 percent of pay in the thrift plan and in many cases get a matching 5 percent contribution from Uncle Sam. Money from those investments can be used for retirement; as loans to meet medical, educational or housing needs, or they can be rolled over to an Individual Retirement Account or another pension plan if the worker leaves government before retirement.

The current contribution rate makes the federal program one of the nation's most generous 401(k) plans. But those rates could be reduced if lower-paid employes fail either to sign up, or to invest the maximum, because of so-called antidiscrimination rules covering 401(k) plans.

Under the rules, amounts that upper income ($50,000 per year or more) workers can invest are limited to no more than 2 percent above the average percentage contribution rate of lower- income workers.

Although figures on employe participation won't be available until the end of the year, indications are that during the first sign-up period many lower income workers either didn't invest, or invested relatively small amounts. If that low-level of participation continues it could reduce the contribution rate for upper-income workers.

Many upper-income workers have been reluctant to contribute the maximum amount because they could be hit with an unexpected tax bite next year if the rate is cut back.

In response to a query from Rep. William D. Ford (D-Mich.) the General Accounting Office suggested several options to protect the thrift plan investment rate. Among them:Require the government to give matching contributions to employes covered by the old civil service pension plan. Now they can put in 5 percent but get no match from the government. Put the federal thrift plan under the less stringent contribution rate rules that cover plans of employes of schools, and charitable organizations. Unlike 401(k) plan rules, regulations covering the so-called 403(b) plans do not base upper-level worker contribution rates on participation by lower-income workers. Rates in those plans remain unchanged so long as 70 percent of the workers are allowed to participate, whether or not they actually invest. Since almost all federal workers are eligible for the thrift plan, that change, which would cost the government nothing directly, is feasible and would protect the contribution rate.

Earlier this year the Federal Retirement Thrift Investment Board asked that the U.S. plan, which it manages, be exempt from the nondiscrimination rules. Ford is expected to push for the changes that would maintain the attractive contribution rate for the federal program.Pension D-Day

Starting next month U.S. workers hired before 1984 must decide whether to stay in their current pension plan, or switch to the new FERS program. It is a major decision because once they switch they cannot return to the old plan.

At 1 p.m. tomorrow on WNTR (1050 AM) Jamie Cowen will talk about the pros and cons of the two systems, and answer listener questions. Cowen, now a private consultant, helped design the FERS program while he was a Senate staff member.