Interest rate on existing mortgage

The answer likely is in the various agreements you signed with the credit union.

In general, one party cannot unilaterally change the terms of a contract with another party without being liable to the other party.

Assuming that your interest rate is fixed, what is the interest rate on your promissory note? Are you getting a discount from the stated rate because of your employment? For example, the note states that the interest rate is 7%. However, because of your employment, the credit union is currently charging you only 6%.

You likely should have an attorney review your agreements and facts with you.

If the note, and any addendums, contain no provisions regarding an adjustment in the interest rate contingent upon your employment, your interest rate should remain unchanged regardless of your employment. Think of it in terms of similar employee benefits. If I am an employee of GM, and I get special pricing on a GM vehicle, if I retire or go work for Ford, GM cannot come back and charge me full price on the GM vehicle unless our contract says they can do that.