UPDATE... First off, thank you all for your comments and suggestions. I thought you'd like to know the final outcome of our plumbing & venting issues.... We have moved the shower drain pipe. After getting a totally unacceptable estimate of $4,800 to move our shower drain pipe & vent from our kitchen contractor that included punching a hole in the roof to vent the drain, we solicited comments and estimates from 3 outside plumbers. Of those 3, 2 proposed the same solution and a third proposed a different solution...none of these solutions involved punching a hole in the roof. Why not? Well, it turns out we have a whole-house vent stack in the attic that we can tap into. All 3 outside plumbers checked our attic to see if we had one and then proposed solutions using that vent stack. Our kitchen contractor's plumber never checked the attic for that stack so assumed he would have to go through the roof. (You know ASSUME does...) Our first outside plumber proposed tapping into the toilet waste pipe. But, all of the other plumbers were uncertain if that would pass code...this estimate was for $550 + $150 for an express permit The other two plumbers proposed rerouting the shower drain back to the main 2nd floor waste pipe...one was for $925 and the other for $1,000 both + cost of drywall repair. We went with the $925 estimate for a couple of reasons...(1) they were local and have an "A" rating across the board on Angie's List (the first time it's been useful!) and (2) the other plumber was not a master plumber (journeyman), he was not insured, and wasn't even sure if his license was up-to-date in our county...all red flags to us. Anyway, so our plumber came out and moved the drain pipe. He then told us he would try to run the vent to the attic without cutting into the wall (saving us the $500 for drywall repair). It worked! He was able to push the vent pipe up the wall and into the attic without any problems. While he was working on this we asked him about the Polybutelyne water pipes...he, as well as all the other plumbers, had recommended changing them out. We're on well water so there's no chlorine involved, but they did tell us that it would eventually become brittle and split. So, we got estimates for that and, since our kitchen project has been stalled for over 8 weeks, went ahead and had it done ($3,200 for replacing water pipes + $1,900 to repair drywall in 3 bedrooms + LR textured ceiling = $5,100. We already have the ceiling down in the kitchen and the main wall opened up so no charge for repairing those) This was all done fairly quickly and we have had both our final inspection done and the drywall repairs completed...in time for our kitchen to restart Wednesday. The only drawback...previously, since each fixture had it's own water pipe, flushing the toilet had no affect on the shower. Now, however....a very different story! It's taking some getting used to! BTW...it turns out the first plumber's solution would have met code and passed inspection since he had planned to use a sanitary T. I was curious so I asked the inspector about it when he was out Friday. If you want pictures, let me know and I'll post them. Thanks again everyone!

Loosen completely the exterior setting ring underneath the shower tub (brass ring with notches at the top of the threads...aka locknut) and that should loosen things up. Step on the tub to release the bottom of the tub from the brass drain. Put a pipe wrench on the brass and turn, don't torque it too hard to avoid cracking your pvc.

Plumbing new drains and vents can be complicated, but in this case it seems you are just replacing your old shower pan and drain with a new curbless design. You are not really changing the basic plumbing design; you are just trying to get the drain to align. I assume you have fully researched the design for a curbless shower, since that is more complicated to implement than your plumbing changes. In most cases its easier to use a pre-sloped pan covered with a waterproof membrane by Schluter or other good company. They recommend also using waterproofing membrane under the bathroom floor for several feet around the shower, since its hard to predict how much water will end up on the floor when you have a curbless design. This company has a low profile P-trap, but I'm not sure it will be low enough for what you need. https://pvcpipesupplies.com/1-1-2-p-trap-low-profile-dwv-fitting-d706-016.html If the vertical pipe in the wall is accessible and made out of PVC or ABS, it is a relatively simple job to cut it and lower the drain connection. If you are motivated enough to break up concrete, cutting into and splicing a drain pipe should not be that hard for you. Bruce

Hi, PVC drain is durable and resistant to most household chemicals including acids. It doesn't corrode. It's tough stuff. I like PVC for drains because I've seen plenty of chrome plated brass traps rot through. (I'd never use PVC / CPVC for supply lines). Copper is pretty durable also. Galvanized iron drain lines, on the other hand, are always a candidate for replacement with PVC. Here's the problem I'm having - I don't like when plumbers splice in plastic pipe (PVC/PEX/Sharkbite) because they don't have the skill to solder a joint. That's just poor workmanship in my book. What's more - when it's supply line - it can inadvertently interfere with electrical bonding and grounding. If they don't want to solder in close quarters next to combustible material that's a valid reason. But if they are lazy or unskilled, that's not at all valid reason.