Releasing the votes

There is no legal requirement that a Senator (or Representative) stick with a commitment he made to his leadership on how they would vote. This is a matter of Congressional courtesy and tradition, not law.

That said, anyone who in reality went back on his word and voted contrary to how he had promised the leadership would pay a heavy price. One of the few things that neither House tolerates is someone whose word cannot be trusted. Once you have given your word to a colleague, you are expected to stick by it. You can't be forced to, but if you violate your pledged word you will find it almost impossible to function in Congress thereafter, as no one will really trust you, and your reputation will have suffered irrevocable damage. This is not only a matter of voting but of any lie you tell your colleagues.

There have been instances where a Sen. or Rep. has gone back to his leader and asked to be released from his pledge, usually due to political fallout back home. More often than not this is granted provided the reason is genuine. When he was Speaker of the House, Sam Rayburn never tried to hold colleagues to votes he honestly knew were politically damaging for them to cast. But he also knew all the districts; so, if a Representative whom he knew would not suffer any adverse political backlash by voting with him (Rayburn) came to him and said he'd be in trouble if he kept his pledge, Rayburn would know he was lying and up to something. He'd usually release him from his pledge, but he made sure to retaliate in other ways.

As to what Munson might have expected when he released his votes, my guess is that he did not at all expect any of them to actually switch: remember his look of astonishment when Smith announced his "No" vote.