[Domo]

Could, maybe, if - those words won't make a perfect fit.

Field engineering is fine for some things, however if that "little project" is what's keeping my home from water damage I think I'd prefer getting parts that are already made for the job.

Are you married? If the answer is yes - I'd heartily suggest you put the right boot on the roof and then install the right pipe. Your future depends on it - after all, "I goofed" is never the way you want to start a conversation with your beloved.

 

[Domo]

True, true!
I think I've got a situation like chandler48 says, where I can cut the rings to accept a larger pipe. If that doesn't seem to be the case I'd definitely wave off. I still have to cut the hole larger from inside without damaging the boot.
Do you think I'm better off to remove the old boot entirely and put a new one on? I don't have a sense of what's involved with that. Do I need to get bunch of chemicals, or just scrape off the old stuff as best I can and smother it with new sealant?

@chandler48 is clever and resourceful but I worry about the UV exposed old boot. In 73 years I don't think I can recall a single task when re-using old material was the best decision. (Yes, I've even straightened bent nails at times and have re-installed baseboards, etc. - but you get my point, I'm sure.)

The boot-to-pipe seal is what will keep water out of your home. Once water gets inside just think of what can be affected. Ceilings get's stained - now you're sealing the stain and repainting the ceiling or worse cutting out mildew and repairing the sheetrock.

The roof is like the tires on your car - that's where "the enemy" can attack the family - either a blowout for a tire or a leak from the roof penetrations.

You sound savvy, but if not confident with roof work, hire a roofer who is licensed and insured and get the top-notch job you and the home deserve.