Just a PSA in case I'm not the only person on the planet (or at least in the US) who didn't know this. If you dump a significant amount of money into your mortgage and want it to be "recalibrated" so that your monthly payment decreases, you can ask your bank to recast it. But you have to ask!
We bought a house with cash, decided to move several years later, and bought the new house with a mortgage a month before selling the old one. It was just a case of finding The One and not wanting to lose it. When the old house sold, we put the proceeds into the new mortgage and paid off 70% of it. I forgot that that wouldn't change our monthly mortgage payment, so our monthly cash flow remained really tight. We could have really tightened the belt and paid off the mortgage in 6 years, but we decided that at our 2.75% interest rate, it wasn't worth living like that. So I started looking into refinancing, thinking maaaaybe we could come out ahead in spite of the closing costs, since interest rates have inched down a little bit more. The guy I talked to said that he was pretty sure it was too late, but it wouldn't hurt to ask about recasting. Sure enough, it wasn't too late! And although I'm sure it varies from bank to bank, apparently even something as "small" as $20,000 is enough to get them to recast your mortgage if you want.
We still plan to pay off our mortgage way in advance, but now it will be on our terms, not on the bank's terms. We'll throw extra at it when we want to, and we'll put the money into other things when that seems more important.
(Edit: inserted a "not" in the first sentence so it says what I actually meant to say)