There has never been a hardforking change of the blockchain protocol rules.

There has however, been a 'hardforking' change of the on-the-wire P2P gossip protocol. A checkvalue was added all messages (negotiated, so compatible) but the negotiation itself got a checkvalue too after a two year time delay (and all nodes switched to the new new negotiation at a hard coded time). It caused a few minor issues but no major disruption.

We've also made softforking changes— changes which strictly reduced the set of acceptable things, so they need some coordination of mining to prevent the formation of forks but don't require a 100% upgrade— and there is one going on right now: https://en.bitcoin.it/wiki/BIP_0034

There are ways you could make increasing the blocksize technically soft-forking change (by having an auxblock committed in the block, and once coin is moved by the auxblock it may never be spent in the real blocks) but it would be hideous and pretextual.  But along those lines, it would also be technically possible to integrate a lower security high volume payment blockchain as a pure softfork.  The idea is that txn would be send to an address on the rapid payment blockchain using an output script that looks like an anyone can spend to old nodes.  New nodes would require that some evidence be presented (via some channel) before accepting a transaction that moves those coins back into the bitcoin network (perhaps a SPV like fragment proving the merge mined federated system agreed with the change?).  (Perhaps by admitting this possibility I can get people to stop obsessing over the block size?  There are a lot of possible things to do here, I personally prefer generally isolated systems because I think that people should have a choice. But even if you demand that it must be part of "bitcoin" that doesn't mean we have to increase the block size)

Oh, and welcome to the forum!