Welcome to the forum!
If the krausen has started to slow down, then it is OK to replace the blowoff with an airlock. However, just to be safe, I would leave the tube on for the next 2 to 3 days anyways. It's cheap insurance against mopping wort off your walls and ceiling. On my last batch I left the blowoff tube on for two weeks and it hadn't even reached the top of the carboy.
There is a very fine line between "hobby" and "mental illness."