So if I wanted to extend a column from 100 characters to 255 characters is this permitted? The fact that I'm not making a change and the BDR kicked me out makes me skeptical.
Off the top of my head I'm not sure and would need to test. There's no specific logic in there for detecting such changes and permitting them, so I suspect not.
If you're changing types in BDR you're expected to do it the long way. Add a new col, update to copy the data, drop the old col and rename the new col. Yes, that's ugly. We'd like to change it at some point. If you find this particular problem annoying enough it'd be helpful if you could send a patch for bdr_commandfilter.c to detect and permit changes that only affect a column's typmod.
In the specific case of varchar columns, personally I recommend just using 'text' and adding a CHECK constraint on length. That's what I do most places, not just BDR.