Here's the standard amortization math, assuming a 6.875% fixed rate, 324 monthly payments left, and principal and interest only (no escrow). After the $50,000 lump sum, the balance is $330,000.
Recasting: the new payment is about $2,243 a month, down from about $2,583. That's roughly $340 less each month, with the same 27-year end date. Total interest from here would be about $396,700. Your lender may charge a recast fee, so check that before you decide.
Keeping the same payment: at $2,583 a month, the loan is paid off in about 231 payments, roughly 19.2 years, or about 93 months (7.8 years) sooner than the 324-payment schedule. Total interest from here would be about $265,100, about $131,600 less than with the recast.
In short, keeping the payment saves the most interest and shortens the loan. Recasting gives you the lower monthly bill and more room in your budget. Whether that flexibility is worth more to you depends on your other finances, and a fee-only advisor or your lender can run the numbers with your exact figures.
