That depends on the kind of change. There is actually a list of changes that are of high impact to the network and block chain but may be implemented nevertheless

https://en.bitcoin.it/wiki/Hardfork_Wishlist

Of course, none of these are major economic changes. To quote the article "Please don't list anything here which would significantly change the committed overall economics of the system, it's safe to assume anything with significant economic impact will _never_ be changed in Bitcoin, because such changes would undermine the trust people have in the system, though they may form the basis of an interesting alternative chain.".

A recent example of a change that affects the P2P network, but not the block chain, is the use of bloom filters to make more efficient (but still secure) light clients possible.