Players using the old DLL's would not have any trouble logging on.. for your server, I would use the zip package and not the exe.. the zip pack has all the one's for the server as well as 1 extra for the game side...
Also, if you are going to run a modded server, you can always include the dll files as part of the server installer.. with old one's backed up of course, and restored on the server pack uninstall.. Mirror Mirror is going to try this to see if stability on the Dyna is going to improve for players.