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When you are replacing a shower floor, it's a good idea to replace the P-trap while it is accessible.
Cut open the shower floor, giving yourself enough room to work around the drain.
Draw lines on the area you want to cut (Image 1), then make a plunge cut with a circular saw (Image 2). Cut along all four lines, then use a reciprocating saw to cut the piece in two (Image 3). Pry up the pieces (Image 4).
Use a reducer to transition from the existing 1.5" drain line to the 2" drain. Apply ABS glue to the pieces and press them firmly together (Image 1). Be sure to place the P-trap in the correct location for the drain (Image 2).