Given the large number of files produced by mosspectra, pnspectra, mosback, and pnback, it is important to understand what can be discarded, what is required for spectral analysis, what is required for imaging analysis, and what must be removed if you change the region (imaging or spectroscopy) or band (imaging only).

There are only a few simple things that need to be remembered.

Changing the band to be used does not change any of the spectral files (*.pi, *.arf, *.rmf, etc.). Those files will be overwritten, but with identical copies of themselves. It will produce an new set of all those files with elo-ehi in their names.
Changing the region to be used changes all the output files except the corner spectra files: (P-fovccdN.pi and PfovfwcccdN.pi). The existing files will all be overwritten. Rename or (better yet) move the files to a subdirectory to protect them.
From these rules you can determine what might need to be deleted (or saved) when you change the band or region of interest. Since the most time consuming part is doing the backscal for each spectrum file, minimizing the re-creation of spectrum files is important!