I know that the 1 1/2 tub drain has to be extended to accomodate the center outlet of the shower and that a shower has a 2" drain. But can an adapter be used to increase the 1 1/2 tub drain to 2" and then extend with the line with 2" and a 2" trap? Or will the complete run need to be increase to 2"? Also will concrete around the drain need to be poured up with tar similar to the tub drain or should the opening be poured up with concrete? The tub is being replaced because the owner is having a knee replacement and won't be able to get over the ledge of the present tub. Thanks for any help!
You can't go from 2" down to a smaller diameter at all, so you will need to replace that tub drain line back until where it is at least 2". A friend had a knee replacement awhile ago and it is now much more flexible than it has been for the last 20 years. But, that is another story.
Important note - I'm not a pro
Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013