I think it would be particuarly unfair if the people who were willing to sacrifice their chars/old guilds created a reasonably successful community, then were forced to share their creation with those who wanted to benefit from a centralised community but sacrificed nothing for it.

If one was created, you'd be forced to exclude pre-existing characters from 'free riding' off the activity of the new community through interaction. Otherwise there'd be no real benefit in being one of the pioneers and so no one would be willing to go through with it. This is especially so if you wanted a community where your characters rank, ability and equipment was earned through constant participation over time. I certainly wouldn't want to make dirt poor and unskilled character, have him gain a reasonable position in the community from hard work, then have someone waltz in from a pre-existing guild claiming to be some immensely wealthy, influential and powerful character who was going to buy us all out. If we started the community with slim numbers, and people realised it was actually a good idea, they'd have to start a new character themselves if they wanted in on it.