I don't have any Altas so I can't answer that part but I do have a BLI loco which I have not converted (and probably never will). With BLI's QSI / Paragon 2 / Paragon 3 you need need to remove the built in DCC decoder which also is the main circuit board for the whole locomotive and hard wire; there was no consideration given by BLI for simple decoder swapping, at least for the Paragon 2 series..
BLI Blueline, from memory contains a built in sound only module but no built in decoder. When you add a DCC decoder to a Blueline you'd normally just add a non sound decoder (8 pin I think) so that the DCC decoder handles all the DCC stuff and the built in module does the sound. If you wanted to put in a proper sound decoder you'd need to remove BLI's built in sound module and hard wire.
Stealth I do not know if they contain a speaker or not but they are non sound non DCC so it could be the easiest to start with.
So, there will not be any advantage to buying future BLI with QSI / Paragon 2 / Paragon 3, in fact it'd be more expensive as your paying for the BLI decoder that your not going to use, though you probably won't have choice as they no longer do Blueline or Stealth as far as I know. Unfortunately this seems to be the way the industry is heading - offering everything DCC installed already and too bad if you need to waste your money on a pre-installed decoder you don't want because you want a different decoder in the model.
- Tim