If you MIRE the loan, you are required to give notice, unless nothing changed and you are within the 7 year period. But, this wasn't your question. Your question was a simple lowering of an interest rate and didn't mention anything about any MIRE circumstances...thus, as Dan said..no notice would be required unless you did more than just lower the rate.
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My opinion only. Not legal advice.
Say you'll haunt me - Stone Sour