I think the answer depends on whether or not you have another vent through the roof (VTR) in the system or not. If this is the only VTR serving the main building sewer, it must be 3" (or 4"!) in order to fulfill the requirement that the VTR is at least the same area as the building sewer (usually 3", but could be 4"). So, if you have some other 3" vent then the only reason to upsize this one is in order to avoid the frost. If that is the case, I believe you can upsize just before exiting the roof (12" as you say). However, if you are upsizing also to meet the area requirement then I have been told this must happen at the joining of two vents (otherwise I guess the benefit is not realized). For example, when your 1.5" bath/kitchen vent joins the 2" it should upsize to 3". Probably you should upsize to 3" and then bring the 1.5" into that.
But... all that said... you'll have to see what the mods or experts say to be definitive. (Also, I'm only familiar with UPC, but probably IRC is the same on this.)