The only problem you could have is if there is not enough Nitrogen in your mix. Most flower nutes are low in nitro and that could cause deficency issues. I mix a flower nute 1-3-3 with a veg nute 6-4-4 to get 7-7-7 or 10-10-10 in veg.
ive never done it this way but im pretty sure you would want like 10-8-8 something with higher nitrogen then when you switch to 12/12 slowly add more p and k and less n