If this were to happen, I bet you ten bucks it would happen on Monday Night on the USA Network. I can see it now "NEXT WEEK, RAW GOES RETRO SMACKDOWN!"
All honestly, when Smackdown reaches a milestone of some sort, I don't know what milestone it would be, maybe 500th episode if that hasn't happen yet. Regardless, when that happens I wouldn't be surprised if they did something special. However I highly doubt they would move it off Friday night for one night. It is like admitting that there product isn't as good as it once was.
As for bringing back the old school set, if it is sitting in storage somewhere in Stanford, Connecticut maybe that would happen, besides that though don't hold your breath.Like mentioned before it would cost too much to rebuild the old school set for a one night special on a Friday Night on SyFy Network.
The only real thing I can see happen for the next big milestone of Smackdown is for them to have a "celebration". A celebration means that the Raw wrestlers will come over to Smackdown and have some matches there, they will recap some of the best moments on Smackdown and most likely people like The Rock and Austin will have taped messages for the audience.