Frame specs say 555mm, if I go to 567mm will the bike become an un-rideable bucking bronco or meh...
Caveat
Im looking at a fun project to put shortened (175mm) dual crown forks on my HT with a slacker 63 as opposed to 65 deg HA. I accept the existing 160mm forks work fine, this is for shits and giggles and cool looks and I dont care a damn for any poor climbing as I walk up anyway, its for hooning around woods and bike parks with the odd very slow weekend away in the peaks.
Head angle might make more of a difference if you change that. You might need to do some geometry to work out if the front end will be much higher.
I'd say go for it 12mm is not a lot and if it isn't bad now it is unlikely to get unrideable.
Last time I bought a frame it was designed for 120mm forks. There is a huge difference between different fork manufacturers on the a-c length. It also changes as the forks work. Guides are just that.
Thanks, my thoughts are slightly longer stem and go for it