http://invest5gh6ln6dom3z2rgtkbctdmpmcotyggb376jtbrfbqp6i2u36yd.onion/bitcoin-global/index15.html
If, for example, a protocol is changed in a way that tightens the rules, that implements a cosmetic change or that adds a function that does not affect the structure in any way, then new version blocks will be accepted by old version nodes. Not the other way around, though: the newer, “tighter” version would reject old version blocks. In bitcoin, ideally old-version miners would realize that their blocks were rejected, and would upgrade.