You can do that kind of change in your own Bitcoin-compatible client, but you cannot be sure that other people will run that version and that it will shut down when you want. Many miners use their own custom software for mining blocks, the same for mining pools. There are many clients that are compatible with consensus, but different than Bitcoin Core.
Also you should notice that Bitcoin community make changes by using soft-forks, not hard-forks. Backward compatibility is preserved as often as possible and there is no reason to change that. Any change can be deployed in a soft-fork way, even "evil soft-forks" are possible, as described in https://petertodd.org/2016/forced-soft-forks. I think that kind of soft-fork is still better than hard-fork.
On 2021-09-12 21:16:00 user James Lu via bitcoin-dev <bitcoin-dev@lists.linuxfoundation.org> wrote:
If MTP-11 is greater than 5 years after the release date of the current software version, the full node should shut down automatically.
This would allow writing code that gives the community ~5 years to upgrade to a version that executes a new hard fork while keeping everyone in consensus, provided the change is non-controversial.