[Xerihae]

I don't think it's quite so harrowing. The number of areas that need to be re-written could be kept to a minimum if it's just things like towns and the odd mention of them in the distance in areas close by. You'd expand the world with newly discovered areas rather than constantly re-writing the old ones. As for the coding, you'd have your basic world set and then a year to develop the extra bits needed to add into it for the next time period. Surely this is no real difference to the MUDs I've seen that work on "versions" rather than continuous changes?

I mentioned that as a possibility and it's not something I personally would add in. However, you could have some form of clichéd cataclysm and just revert back to the previous version of the game because no-one advanced the game into the next period. This would require no work at all really.