This is very region dependent. But here in Canada we have 25-30 year mortgages broken up into 5 year terms, every 5 years we renegotiate our interest rate and have the option to switch lenders.
If your house value were to drop 75%, it might make it harder to switch lenders (does a bank want to lend you more than what the house is now worth).
I think there is an escape hatch here where if you stay with the same lender they will still accept you, but I honestly don’t know much about the specifics.
Things would need to get pretty bad for this to actually matter (and I suspect the government might step in if it became widespread)