Disclaimer: I'm no expert on this topic and Section 527(a) of the SCRA may have been interpreted to mean something other than its plain language.
If I'm reading Section 527 correctly, there should be no need to recalculate anything if you have been in compliance since you received the servicemember's written notice required by Section 527(b). Ignoring 1098 issues, it appears that you would treat the loan as if there is a lender-funded buy-down of the interest rate to 6%. The total monthly payment you post should stay the same but the borrower is only required to pay the amount that remains after you calculate and post the forgiven interest. When the servicemember is no longer eligible for the interest forgiveness, you simply stop subsidizing the interest. This approach leaves the outstanding principal balance the same as it would have been without any interest rate relief.
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...gone fishing.