The service member has up to 180 days after their active duty ends to notify the bank in writing they wish to invoke their rights under the SCRA. The bank will have to make the required reductions retroactive to the date they entered active duty when they make the request, even if the loan has been paid off during that time.
According to the above cite you cannot require acceleration of principal; and applying the retroactive adjustments to the account would IMO be doing just that, therefore you would have to make the appropriate reimbursement to the borrower unless they agreed in writing to have it applied to the loan (waiver under 517). However, I want to emphasize that is my interpretation of the above cite.
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The opinions expressed are mine and they are not to be taken as legal advice.