Quite true.  In fact even more so because "old" protocol nodes will only accept small blocks, while the "new" protocol nodes will accept either small (<1MB) or large (>1MB) blocks.  Thus all blocks produced by old miners will be accepted by the new ones as valid, even when there's an extra 500KB of transactions waiting in line to be published.

You'd need like a >90%, simultaneous switch to avoid total chaos.  In that case substantially all the blocks published would be >1MB, and the old protocol miners wouldn't be able to keep up.  If normal nodes switched at the same time, they would start pushing transactions that old-protocol clients / miners would lose track of.  It seems very likely that when / if the change takes place, blocks will have been at the 1MB limit for some time and the end of the limit would immediately result in 1.5MB blocks, so it would have to be coordinated well in advance.