"We can change the terms at our discretion" is a line that gets the book thrown at you in court, at least in EU, and that's the start of the humiliation conga for whoever tries to claim such a clause.

You cannot insist on a clause that lets you change the contract at your discretion and having the resulting amended contract be valid without acceptance by the other party.

You just explained why EU software startups do not cause VCs to rain money down on them.

I've seen a lot of bad takes about this topic, but "there's no VCs because you can't ransom your clients for extra money" is... out there, special extra.

You can raise prices, you're just not guaranteed that a client will remain your client, and when you change the terms of the contract you void any contractual penalties as well.