FWIW, we are doing a similar thing for a borrower, taking a loan from a 30-yr fixed at one rate to a 3-yr int only at a lower rate. What we are going to do is include in the new note (probably under "additional terms") langage that says at the end of the 3-yr int only period, bank will execute a second modification, returning the loan to P&I payments and extending the maturity another 36 months. Our system won't support it being done automatically, so we'll have to touch it again in 3 years.