The transactions that generate coins illegally will always be considered invalid by you, even if you are the only node in the network that believes this. If a change to generation rules becomes overwhelmingly popular, then it might reduce the value of your coins to nothing. The system will still work for you, but your coins won't have any conversion value.

This isn't going to happen overnight. If a new Bitcoin-like system continues to generate 50 BTC for every block after 210,000, then half or more of the market will be "old BTC", and half will be "new BTC". There will be an exchange rate between them. Probably "old BTC" will not die easily.

There is no technical way for a full network node to be tricked into accepting an illegal generation. It's clearly invalid.