splicing a 25 pair telephone riser
Firstly, I don't know what your specific situation is, but it sounds like what you're trying to do might be illegal. You're potentially messing with people's emergency services (911 access) and Verizon might own the cable you're trying to mess with. But, I'll assume you've worked this out somehow, and take no responsibility for your actions. 

I used to work on telecom and have done cutovers on live copper. It's easy; much easier, but dirtier, than fiber.

Moving to a larger gauge wire is not ideal, mainly because splicing it together could be less reliable (the UR2 is designed for the same gauge wire), but otherwise will not cause any issues. 

Using a temporary connection while cutting over is standard practice. We used some vampire clip jumpers (AKA ABN (Angled Bed-of-Nails) clips) to make a temporary bridge, cut out the old, made a new splice in the middle. The gel inside the UR2 (icky-pic) is to keep water out, not act as a conductor, so the vampire clips are just as good in the temporary situation. If someone is talking on the phone during the cutover they will probably hear something, but should not be too disrupted.

I encourage you to check the law on this. If you don't have permission and licensing then even if you're not caught in the act and Verizon later finds out someone illegally performed the exact thing you already got a quote for... Well, it's still illegal and they have a damn good idea who did it.