Are there proposed hardfork changes that couldn't be made if the market was filled with ASIC miners?
As long as no changes are done to the structure of the block header, the ASICs should work just fine. There are some proposed changes that could alter it, like protecting the block chain against pre-mining, or adding a new field to the block header.

So yes, when the ASIC miners become the majority it will be hard to push those changes unless Bitcoin faces some serious issue that forces the change (for example, an attempt at a 51% attack).