I am planning my plumbing for a bath remodel. The bath uses a 1-1/2" galvanized pipe drain connection that goes into the 4" main iron drain line via a 4"x2" lateral. The joint appears to be soldered. By code I need a 2" minimum drain line for the bath in order to accommodate an additional separate double shower.

I am wondering whether I should try to take apart the 1-1/2" connection and replace with a 2" or bust the 4" cast iron, boot in ABS pipe and glue it up.

How tough is it to undo the joint and replace it?

Let's see if I have it right. You have a 1 1/2" galvanized drain that runs from the tub into a 4 X 2 Sanitary tee. This galvanized pipe is connected to the branch hub of the tee by a lead and oakum poured and caulked joint. Am I correct?
I can save you the hassle of tearing up your stack. First let's get the pipe out and the hub cleared. To remove a poured lead and oakum joint take a 3/8" drill bit and drill a few holes down past the lead and into the oakum.. Push a screwdriver under the lead ring and lever it up. Once it's free you can began to peel the lead ring out. Now pick out the oakum and you can pull the joint apart and clean up the hub.
OK! Now whadda we do? We transitionfrom cast iron to 2" PVC using a 2" Duel-Tite Gasket (see image). You will also need a can of Duel-Tite Lubricant or some heavy axle grease and a short piece of 2" PVC about a foot or so long. Insert the neoprene gasket, into the hub. Do not grease the outside of the gasket before inserting. Now take a file a file and bevel the edges of PVC smooth, so the pipe won't hang up on the ridges in the gasket. Next lube the pipe and the inside of the gasket with Dual-Tite Lubricant or a heavy grease. Push the pipe in, making sure that it's lined up untill it seats on the bottom of the hub. We use a Johnson Bar for leverage but you can use a 2X4. If the pipe's difficult to start it's OK to trim back the first few ridges of the gasket with a sharp knife. You may now purchase a small can of PVC glue and primer and a 2" PVC couplimg and complete your drain. Good luck, Tom