You will not be on the main chain if the modified version changes some consensus rule. That's called hard-fork. Look at the segwit2x debate to see how this work

You will not be on the main chain if the modified version changes some consensus rule. That's called hard-fork. Look at the segwit2x debate to see how this worked in real world. If a modified version doesn't follow the same rules, it will never finish the initial sync. And you can still sync with very old clients. Most of the work on the bitcoin client is optimizing validation. Consensus rules are rarely touched.