Pro Logic II automatically decodes the old Dolby Stereo into the four channels and Pro Logic II into 5.1. It doesn't add anything else to the old Dolby Stereo. I have both decoders on my Yamaha, but there isn't any difference playing Dolby Stereo through both. I'm curious if Dolby Surround just intelligently applies the decoding depending on the signal, knowing whether it should be 4 channels or more. It would be fine if it just spread the same four channels over more speakers, but it wouldn't be good if it is performing additional processing, like splitting off bits of channels for the additional speakers or adding phase and reverb tricks that aren't part of the original mix. It's easy to check for that by just playing a Dolby Stereo encoded track with Dolby Surround activated and stand in the rear of the room. All the sound you hear coming from the rear should be the same, not divided into left or right or Atmos elevation.
I suspect that they just put it all in one button and it figures out what the intended codec to use based on the signal. It probably still decodes Dolby Stereo, Pro Logic and Pro Logic II, it just doesn't tell you which one it's using.
I've gotten into the habit of checking the old Dolby formats with movies from 1975 to 2000. Often the Dolby Stereo encoding is intact and is better mixed than the modern 5.1 mixes. Maybe selecting the stereo track and engaging Dolby Surround will give you the same result.
One other thing I've noticed is that some music albums, beginning in the mid to late 80s, seem to be mixed for Dolby Stereo. Engaging the DSP puts the vocals in the middle and adds effects in the rear channel. It makes no difference for other albums. There were albums that were specifically released as being in Dolby Surround (Tomita in particular) that were re-released on CD without the Dolby logo, but the master is clearly encoded in Dolby Surround. These are encoded identically to the Dolby Stereo movie format.
http://www.surrounddiscography.com/dolby/dolby.htm
By the way, I highly recommend the Charles Gerhart / National Philharmonic classic film scores CDs. They sound great in surround.