Yay - a fun way to spend some time on your engine. A friend of mine did an engine swap and was in the process of replacing the freeze plugs as well, only to find there was already a loose one floating in the block (well, what was left of it) That was fun fishing that out too
Having one disappear into the coolant gallery isn't a deal breaker but... it is probably a good idea to make sure they're all accounted for