If it is a stereo tape, DVD, LD, broadcast....whatever and it was a movie mixed in Dolby Stereo, it will always be in Dolby Stereo, regardless if that little logo is on there. The way Dolby encoding works is it puts the surround information in the out-of-phase content of a stereo signal (L-R). Everything that is stereo has out-of-phase content. Play any stereo CD in Dolby Pro-Logic if you don't believe me. All Pro-Logic is active matrix decoding (as opposssed to passive decoding, just known as Dolby Surround). To get the center channel it sums the left and right channels (L+R). And that's it! So there is no need to worry over any of your stereo videotapes, or broadcasts, or DVDs or LDs.
And Pro-Logic is a decoding process, not an encoding process.
Neil