Why not pop the cover and monitor the temperature of the transistors at the designed frequency and then bump it up to 468 and monitor again. It it starts getting hot compared to the baseline temp curve, quit. One thing is for sure, if it shouldn't be done, there will be a mismatch at the transistor, and if that mismatch is bad enough, a finger on the transistor will let you know.
Maybe you could use an RF voltmeter and see what the RF voltage is at the transistor at its designed frequency (into a 50Ω load of course, not the 468MHz antenna) and then switch over to 468 and tinker with the matching network until you get the same RF voltage as you did at 450...
You can always put the smoke back in...