If this backward incompatible change will have to be done one day, why not make it only once (by setting an automatic adjustment rule), and why not considering making it now, while it's still easy?
Patches welcome.
But be aware that any patch that is vulnerable to denial-of-service attacks will be rejected, and I can't think of a way to automatically adjust the block size that wouldn't be vulnerable to some big, anti-social miner (think "jerk with a botnet") deciding it would be fun to artificially drive up transaction volume, drive up the block size, and create a few gigabytes of worthless blocks we all get to download forevermore.
When we get close to bumping into the block size limitation it will be easy to convince a majority of the network to upgrade-- that's one problem that is obvious and easy to fix.