We bought a 1928 house, the plaster is sagging here to the point it is flush with the trim. How should I go about repairing this? Is it worth fixing? Thanks
There is a strong chance that what you see is drywall overlaid a plaster wall. Plaster, does not sag or bow out as it is shown in your picture, not without severe cracking. Plaster is very brittle and does not take well to any movement at all.
Based only on what the picture shows, the plaster did move, or cracked or fell off or something like that. The remedy that somebody thought of, which is done a lot, is overlay the wall with drywall, whether it be 1/4" thick or thicker.
Flat window trim in homes the age your is typically very thick, 3/4"-7/8" thick, unless it was redone with newer stock. Regardless, the trim's edge is covered to a degree by the wall material, suggesting an overlay of drywall over the plaster.
To answer your question, since the base is already off, so it appears, cut the offending section out to the original wall surface, remove whatever is creating the waves in the wall and re-shim if needed and reinstall the same thickness of new drywall that was removed. It will still cover the trim to a degree, but it will be uniform.
The only option would be to replaster it.
Plastering requires great skill and in old houses you will often find plaster jobs done by run of the mill builders who are not really plastering artisans. Therefore, over time you have problems such as the one pictured above. Nowadays, it is practically impossible to find skilled plasterers, so a typical contractor will just want to replace it with drywall, which is much, much easier to install than real plaster.
What you have is most likely a bulge of drywall mastic that is covering a spot in the real plaster that likes to crack. You could sand it down, only to find yourself putting it back the way it was when the crack develops again. Personally I'd caulk it (the trim) and be done with it, but I deal mostly with rentals. If in my own home, it'd be dealt with when the plaster gets pulled off to do new insulation and re-wiring.
A large bead of caulk where it isn't so close to the wall would alleviate some of the shadow and make it 'go away'; be more uniform. It would be less of a concern if your going to use the same paint on the walls as on the trim.
Is it worth fixing, to you? Slap a coat of paint on the whole thing and take a look at it. Keep in mind no one will ever look at it as hard as you are right now.
If it really bothers you, replace the trim board with one that has a larger edge. The one you have there has a beveled edge that basically makes it look like it disappears in some spots (or someone hit it with a sander when they did that patch). I'd be harder to tell that the wall was so wavy if the trim had more of an edge to it.
Beyond that, you might as well gut the place.