Given the pretty generic way manufacturers tend to size bikes these days (roll eyes) as calstar says it may be about 6 cm shorter in the seat tube.
So if your 56 has a whole lot of exposed seat post you might be in a little strife...but if you're running a pretty small seat to bar drop on the 56 the 50 might be doable with a longer seat post and longer stem. You may even need to flip the stem over to angle up (shudder)...