You would be losing some sidewall there. They'll ride stiffer cornering but you would be more likely to get a pinch flat or bend the lip of the rim. I've always had 12" wheels on ATV's before and just bought some 14" but going to run a 28" tire.
Looking at the wheel and tire combo's radius..
On 12" wheels with..
25" tire = 6.5" sidewall
26" tire = 7.0" sidewall
27" tire = 7.5" sidewall
With 14" wheels..
26" tire = 6.0" sidewall
27" tire = 6.5" sidewall
28" tire = 7.0" sidewall
30" tire = 8.0" sidewall