First, prepare your workspace. Place a thick, dry rag or an old towel over the tub floor around the drain to prevent any accidental scratches or damage to the tub's acrylic surface. Next, remove the existing lift-and-turn stopper. To do this, open the stopper, then locate the small set screw on its base. Use an Allen wrench or a small flathead screwdriver to loosen this screw. You do not need to remove it completely. Once loose, unscrew the knob cap from the top of the stopper post, and lift the entire stopper mechanism out of the drain. Clean out any hair or gunk to get a clear view of the drain flange and its crossbars.

Since the flange is stuck, hardened plumber's putty and corrosion are likely culprits. Apply a generous amount of penetrating oil, such as PB B'laster or WD-40 Specialist Penetrant. Spray the oil directly onto the seam where the top edge of the flange meets the bathtub, allowing it to seep down into the threads. Also, try to spray some directly onto the threads from inside the drain. Let the penetrating oil sit for at least 30 minutes; for a very stubborn flange, allowing it to penetrate for several hours or even overnight can make a significant difference.

The most effective tool for this job is a specialized drain key, also known as a dumbbell wrench or drain remover. Select the end of the wrench that fits snugly into your drain opening. Insert it so the lugs on the tool firmly engage with the crossbars inside the flange. Once seated, place a large adjustable wrench onto the hex-shaped body of the drain key. Alternatively, if your key has a hole, insert the shaft of a large, sturdy screwdriver through it to use as a T-handle. Apply steady, strong, counter-clockwise pressure to break the flange free. Avoid jerky movements which can break the crossbars.

If the flange is still stuck or the crossbars break, you must resort to a more destructive method. This is the point of no return for the old flange. Put on your safety glasses and protective gloves. Using a Dremel-style rotary tool equipped with a reinforced metal cutting wheel, carefully make two vertical cuts on opposite sides of the flange's interior wall. Cut from the top lip down towards the threads, but be extremely careful not to cut all the way through into the drain pipe threads or the surface of the tub. The goal is to weaken the flange. Once the cuts are made, insert a flathead screwdriver into one of the cuts and pry the section inward, toward the center of the drain. This should break the flange's tension and allow you to remove it in pieces. If you don't have a rotary tool, you can wrap a mini hacksaw blade in a rag to use as a handle and slowly saw through the flange wall, though this is much more difficult and time-consuming.

After successfully removing the old flange, it is crucial to prepare the surface for the new installation. Use a plastic putty knife to scrape away the bulk of the old, hardened plumber's putty from the bathtub surface around the drain opening. Use a wire brush to thoroughly clean the female threads inside the drain pipe (the drain shoe), removing any bits of old putty, sealant, or corrosion. Be meticulous with this step to ensure the new flange will seal properly. Finally, wipe the entire area—both the tub surface and the pipe threads—with a clean rag. For a perfectly clean surface, you can wipe it down with a rag dampened with mineral spirits or rubbing alcohol to remove any final oily residue.