The former.... I did exactly the same thing before a product transfer with same mortgage provider. I didn't even bother to tell them, on the day my old deal expired they took the end of day balance and started that on the new remortgage on day 1.

They usually adjust balance and refund the overpayment.

I have an offer from my bank for a new product switch for later in the year which obviously has the loan amount set to what I will owe at the end of my current fixed term (£150,000). If I make a 1 off overpayment (£10,000) now before my current fixed term ends and the new one begins, will the bank just automatically adjust the total loan amount for the new product or will I need to apply for the entire thing again?
Sorry if this is obvious I've just found it very confusing! TIA!

What happens if I overpay mortgage before product switch