Even if there were a button, mb only recognises the change if the track is restarted and then the next track.
No, it will play the current track using the current setting, and will apply the new crossfade condition after the next song has started.
To me it seems a valid request that isn't likely to cause other issues.
@My Altered Ego
It's very unlikely that this would be added for the bottom player panel, since then that function would also need to be made available for all 3rd party skins.
(I would love to be able to create skins that have non-standard buttons and functionality, but that would be a very different topic)
So if your request is fulfilled, it will be a keyboard shortcut for which you will be able to create a toolbar icon at the top of MusicBee's panel.