Anyone stuck a 120mm fork on a Grand Canyon hardtail and lived to tell the tale? Asked canyon the question and got the usual ‘tested with 100mm only’ ‘invalidate the warranty’ answer.

It’s only 20mm extra. What’s the worst that could happen?

The worst that could happen is that you could rip the head off and die as a result

As a rule of thumb people usually get away with a small increase, with the caveats on changes to head angle, BB height etc.

Although you may wish to consider why you want to change the forks, if it’s for more aggressive riding then maybe the entire frame or wheels will be the weak point.