Yes, one can maybe upmix the original 2 Ch AC3 in 5Ch Ac3, and be sweet is a rather good choice.
The "upmixing" process means that a DolbySurround decoder extracts the matrixed infos from the two channels and re-encode them as separate channels on to a new audio Track.
Standard 5Ch AC3 use 384K while 2Ch uses 192 so space used is doubled.
Simple process means you get exactly what your AV receiver still gives you making it on the fly, unless you tweak the "unmatrixed" channels even more (delays, eq...).
My receiver does a good job with properly encoded material so I found pointless to tweak with upmixing.