If I keep the old map:
People are happy for a while, but then as time progresses start to get mad at me for not having a good map that supports all of the new features and awesome game material that is coming out in 1.8. In a matter of a week or so people will be asking me for a new map or to move the spawn.
By now people will probably be a bit tired of the ridiculously huge snow biome that spans for a good 500 - 2000 blocks in each direction. So, I'll have to build a spawn that is far a way to remove traces of that map that we are all sick of. At this point, the map will be generating new chunks, and these new chunks will contain the new content from the 1.8 update. This way, people can go to their old houses if they so choose to take that journey. Unfortunately, this will make the map ridiculously huge. It will be like 2 maps combined in one huge map. This will cause the server to lag a lot when loading chunks and when the server backs up. People get mad when the server lags... Very mad. Eventually, the server map will be so big that it might just be in the best interest of the peoples to reset the server map anyways, but this is an extreme scenario.
The server peoples will be very mad at the beginning, but in a few weeks we'll forget about the old map. We will have all the new content and we can have a nice fresh beginning.
-Mr. Z