Plastic pipe is solvent welded (think the glue you may have used to assemble models when younger)...it melts the plastic to weld it together. So, yes, you need to (generally) cut it off. If you have access from below, you'd want to cut it off a ways down so that you could install a coupler and a new riser pipe. An alternate would be to cut it off then use a Rambit, or similar tool which is designed to ream out the socket the pipe fits into, so you can reuse the elbow or other coupler then just install a new riser. You can get an inexpensive inside pipe cutter to cut the pipe. It's essentially a small saw on a shaft that you use to cut the pipe from the inside.
If you cut the pipe just under the flange, you won't have enough room to install the coupler required to put in a new piece of pipe without the new flange being way too high.