Well, I had idea it would happen, but now I'm certain.
About the current server maps.
The creative map won't have any alterations of what it currently is, but the creative mode will be no doubt a lot better and without a load of server mod and client mods, like true classic creative mode, so there are no worries there, the map will stay as it is.
The Realms map will have a few modifications, mainly the spanw point will move (and leave the current) into a newly generated position, of course atm I have no idea where, as I will have to travel he map to look for a nice location, so, the current work will not be touched or thrown away, instead the spawn will move into a new location, keeping both spawn points active won't happen, so by the time of the update, when you press spawn, you'll travel into newly generated maps.
About mods and mmo levels, I have no idea how they will behave with the adventure update, factions and claimed territory will for sure work (i hope) as economy, Notch hasn't touched the subject, so I guess the vanilla server won't have economy support leaving that option for modders.
The game is looking pretty sweet and I can't wait to start updating our servers with 1.8
As usual, every update brings downtime and problems with mods and plugins, so in a couple of weeks the 1.8 will be released and I expect minimum downtime as I tend to keep the server well up to speed with releases.
maybe you should start thinking about wiping both maps tunes that way the newly generated maps includes rivers,canyons mine shafts and the bigger dungeons its isnt needed on both but it be cool to have the new maps from scratch on the survival sure we would have to start over but it will be fresh adn its nothing nobody can recover from in a short amount of time