There's 2 kind of fixes.
The easiest one :
The game's param.sfo says it require 3.50, but the developers used an older SDK (3.41 for example) to create the game.
In that case, don't do anything anymore.
Just use a recent Backup Manager (Gaia Manager 1.03.1 for example), there are options in the loader to patch the param.sfo when launching the games.
It will replace the requirement firmware with the one you are using.
The tricker one :
The developers really create the game on a 3.50 SDK, and it include a new encrypting key that your current firmware can't decrypt.
You have to pray that the developers released an update on PSN developed with older SDK (3.41).
When installing the game update, it will replace the unreadable encrypted eboot.bin with the new readable one.
You can then launch the game the usual way.
And that's all.
You can't play other 3.50 games.
there's no hack or patch to apply.
It's either a false 3.50, or there are