Hello andyfd,
If you have just accidentally selected a different coordinate system (for example, incorrect UTM projection) for the chunk referenencing, you can switch the coordinate system via Reference pane settings dialog (providing that the coordinates of the reference points in the Reference pane do actually correspond to the new system that you are switching to) and then press Update button on the pane to apply the reference data to the processed data (tie point cloud, dense cloud, mesh and tiled model).
DEM and orthomosaic should be rebuilt, as they are considered as isolated processing results. However, in principle, it is possible to switch the coordinate systems of DEM and orthomosaic via Python, but I wouldn't guarantee that you will get the desired result. But on your own risk you can try the following commands input one by one to the Console pane, after switching the chunk's coordinate system to the proper one:
chunk = Metashape.app.document.chunk
chunk.orthomosaic.crs = chunk.crs
chunk.elevation.crs = chunk.crs