Im trying to get my amortizing spreadsheet to work but its turning out to be more difficult than I first thought

I have a (theroretical) loan of 100k @ 6% for 30 years- P & I. Repayments would be $599.55 per month.

After 5 years, the interest rate jumps to 8%

Is the repayment then going to be $733.76, or $718.21 p/m (or any other figure?)

How is this new figure calculated?

Im thinking that its calculated on the original loan amount, rather than the remaining balance at that time, and over the remaining years left of the loan.

Anyone?