Now it's 3/4 ALL THE WAY!
I finally bit the bullet this weekend. I heated up the old tee and took it out, no problem. Somehow I was able to get enough indirect heat on the new copper fitting (from the extremely small access point I had to work with) to melt some solder (I made sure I used about 6" of solder just to be safe, since I couldn't see the joint and I could barely fit the solder back there).
I did have to (blind) shoot the fire extinguisher a couple times into the wall cavity, though!
Presure test worked, didn't burn down the house
, and now since it's all 3/4" I don't have to worry about
any restrictions (even if it's not a dramatic change, it always would have been in the back of my mind that it could be BETTER!)
Thanks so much for all your suggestions!