It depends on the type of window replacement.
For a “pocket” or “insert” replacement, where the new window frame fits inside your existing frame, you usually don’t need to remove interior trim. This is the most common and least invasive option for standard replacements.
For a “full-frame” replacement, where the old frame is removed down to the studs, interior trim typically does need to come off, at least on the side being worked on, so installers can access and properly seal the rough opening.
Full-frame jobs are more common when there’s rot, water damage, or you’re changing the window size. If your window and surrounding frame are structurally sound, ask your contractor whether an insert replacement is an option to avoid disturbing trim.
Can You Replace Windows Without Replacing the Frame?
Yes, this is called a pocket or insert replacement.
The installer removes the old sash and hardware but leaves your existing frame in place, then fits a new window unit inside it. It’s faster and less invasive than a full frame replacement, since there’s no need to disturb the exterior siding or interior trim.
The tradeoff is a slightly smaller glass area, since the new unit sits inside the old frame. This option only works well if your existing frame is still solid, square, and free of rot or water damage. If the frame is compromised, a full frame replacement is the better long term choice.