I agree with Andy - you need to look at the intent of Reg O here, especially the provisions in 215.43 regarding extending credit on substantially the same terms "including interest rates and collateral" as those offered to noninsiders.
The fact that all employees are eligible for modifications is less important than the fact that the rate to which the loan is modified is either one offered to noninsiders. As an alternative, the rate could be one established under an employee benefit program for which all employees are eligible.
Fianlly, remember that Reg O is probably the most sensitive regulation for compliance. Anecdotal evidence from the supervisory agencies indicates that insider abuse is a factor in 75% of institution failures. The regulators are not going to tolerate insider preferential treatment, because they see it as an indicator of a troubled bank.
You would be well served to apply the Reg O standards and requirements to modifications, even though it would appear that the definition of an extension of credit may not cover modifications. I think your regulator would consider a modification the "making or renewal" of a loan.
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The opinions expressed here are personal and do not represent opinions of my employer.