Buyer talking about changing terms of promissory note later

[Jeremy Lee]

Hi,

My in-laws have signed a promissory note to privately loan $60k to the buyer of their restaurant over the course of 2 years at an interest rate of 5.4%. The buyer recently told my father in law that she might be interested in shortening the term of the note and pay it off sooner. 

I'm guessing this isn't an uncommon situation but was curious how shortening the term of a private note generally impacts the interest rate if at all. Do sellers usually raise the interest rate in the case that a buyer wants to pay-off a note sooner than the agreed-upon date? Or do they just take the difference of what was paid and what remains and calculate based on the 5.4% interest in this case (or re-calculate based on that interest using an amortization calculator, etc)?