Increasing the maximum block size would cause a hard blockchain fork. That sort of thing has been done before on a couple of occasions but it's by no means a small deal.
When I wrote my comment I was also thinking of https://bitcointalk.org/index.php?topic=827.0 from 2010, but on a cursory glance it looks like that was resolved with a soft fork, which is possible because the rules were made more restrictive rather than more permissive.
... which wasn't a hard fork. The only hard fork so far was the move from BDB to LevelDB for chain state data.