OP, could you do this? Sure you can. But why would you want to? If your setup sounds great, why change it? If you are attempting to free up amp channels (which is how it sounds to me), why not just buy an amp with more channels, or another one all together? If it's a money thing, I get it. But you'll be spending either way. Additional amp channels, or rear speakers, labor hours, etc...all cost something. Does your 3 -way run the GB10 or 15?
IMO the easiest way forward is just to experiment. Disconnect the 25's and try tuning it as a 2-way. If you can get it close to how it sounded before (enough to be happy) then you've answered for yourself whether or not it can be done like you're asking.
The difference in tuning 2-way versus 3-way? One more speaker per side. LOL! Tuning is tuning. It's the same whether you have 2-3-or 20 speakers. The time it takes to achieve the desired goal is the only real difference. There is a reason it's referred to as an audio "system." The components you select either work together to produce the SQ you want, or they don't. And how easily the tuning is of the selected components really boils down to skill level of the tuner, nothing more. Since happiness with the final product is subjective, it's your call to make.
Well, why change my setup? For me, half of the fun is enjoying the music, the other half is trying new things with my sound system. In the past, I tried things without asking here and end up loosing time. You know, as a newbie in this hobby, I lack knowledge and experience to have a good judgment or instinct so I better ask or search here when not sure.
Concerning the tuning of a 3 way vs 2 way, I can tell you that it wasn't so hard to get my 3 way sounding very great with the help of tuning guides I found here.
Anyway, thank you guys to take time to explain the probably obvious for you!