Is it against the rules? : No, there is no "rule" that states you are unable to hot-swap a minion for a more powerful statblock.
Is it a good idea? : No. Or, more accurately, probably not. There is always the unspoken "social contract" where players and GMs can assume that both parties are being honest with each other. Also, as DM, you should always avoid "railroading" as much as possible. Be flexible.
You play the game to play a hero. Or at least an awesome 'larger than life' person. So roll with it and let the player feel awesome.
You can always bump up the obstacles going forward to provide more of a challenge. You can insert things ahead of them (or even behind them) that they are/were unaware of. Roleplaying is an interactive adventure, so interact with your players. Help them interact with your world, let them make their mark. Let them feel heroic.
That doesn't mean they need to be playing on 'easy mode', though. Games are much more interesting when they are challenging, so keep on challenging your players. Just don't start pulling bait-and-switches on them to keep things "on the rails".