I'm not sure that you understand what's going on exactly. The area inside the city is corrupted, we don't know what's causing it yet, though it is possible it could be mods, but if you go too far into the city, you will lag to the point of not being able to play, and then be disconnected from the server. The reason we're wanting to change the spawn point is because the area around spawn is so laggy it's almost impossible to get away from it. This will be the reason we change maps if we find that there's no way to fix it.
Not to mention that it has been reported to be spreading to the other districts. Of course it isn't going to spread if the chunk isn't loaded, but if it is spreading then going into the city could make it spread into other chunks. We are still not sure if this section of the map is the only corrupted section or if it is just coincidence that it appeared in that area. If there is more than one chunk that is corrupted then we may never know when we'll run into one.
It very well could be version 1.5 itself doing this, it wouldn't be the first time that major bugs have come with an update. If it is 1.5 then the entire map may corrupt over time. We'll just have to keep an eye on Notch's blog and twitter. Last time we switched maps it was because of some "personal choices" that were made but unknown to us was the fact that the newest update at the time required a map switch. A map switch
may be required in order to be fully compatible with 1.5 but that is unlikely or Notch would have said something.
Then again, he isn't the
best developer.