If you have an account rep then you can get it all down on a spreadsheet and then switch old one off and new one on simultaneously.
All historical data will still be in old account, but all the love (or not) your keywords got will be remembered by Google on the new account, so no downside in performance.
We did it recently and it worked very successfully. All the new work could be done offline, prior to upload.
When copying into Excel, be careful to remember to redo your phrase terms, as the quotes are not copied over. I don't know why that is.
If copying from another application, this is how you can preserve the quotes in excel:
1. After pasting, a little clipboard will appear at the bottom of the paste. Click on it and select "Use Text import Wizard"
2. After fixing the first screen you'll see, hit next. At this point you'll see a " with "Text Qualifier" next to it. Change the Text qualifier to {none}.
Then just hit finished. All your quotes will be preserved.
If you have access to the bulk upload / bulk outpost features, reorganizing accounts within an excel spreadsheet is fairly easy.
What WebDiversity is referred to is 'linking' accounts. This is when two accounts reference the same historical information. It can also be used to manage keyword or campaign lists which are too large to fit into a single account.