Doors and Windows - Replacing entry door and frame between sidelights

We need to replace our entry door and frame around the door. It looks as though when with house was built the door and sidelights (one on each side of door) were installed as one unit. Is there anyway to replace just the door and door frame since there is nothing wrong with the sidelights?

You usually can replace just the door and frame, but not always. The door frame and sidelight frames are usually three separate frames that are mulled together with corrugated fasteners, mull trim, and sometimes glue and caulk. If only the center door frame is bad, you would have to uninstall all three since they're connected together as one, pull out or cut out all fasteners that mull them together, put the new door frame between the sidelight frames, and mull them together. Then apply new or the existing exterior trim (usually brickmould) and reinstall the door.

Some other thoughts:
1. The new door may have a different sill than the sidelights; if that's important to you it may dictate what brand you will be able to select.
2. If the sidelights have a panel below and are only half or 3/4 length glass, usually the panel below the glass aligns with the door panel. If a different brand is selected other than the original door brand, the door may not perfectly align with the sidelights. It's not necessary for them to align, but it's nice to have.
3. Now for the "usually' part. There are some door/sidelight combinations out there that had one frame for all three units: one frame to fit the door and two sidelights rather than a frame for each sidelight and door. If you have that type, you can't replace just the center frame. It is a cheap way to make them, and I only see it about 2% of the time.