Might getting a molar removed mess up the work my braces did?

Posssibly: If you do not have the tooth replace, there is a good chance your teeth will shift and undo what the braces fixed. If the tooth can not be fixed and must be removed, a fixed bridge or implant can be used to replace the tooth and not undo what the braces did.

Usually not: It really depends on which molar you are having removed. If it is a wisdom tooth there should not be a problem. If it another molar it probably won't mess up the orthodontic treatment but it will affect your chewing ability and may allow the adjacent teeth to move into the space created by the removal.

Possibly: Removal of a 3rd molar (wisdom tooth) will usually not affect your Orthodontic result. Removal of a 1st or 2nd molar, without replacement (usually implant/crown) may result in Posterior Bite Collapse...ask your Dentist to describe. Wearing your post-Orthodontic retainer(s) can help stabilize teeth until replacement accomplished. Discuss this with your Restorative Dentist and/or your Orthodontist.