I was wondering about this for some time now, and have never quite gotten around to asking. This is for my site at http://www.rangerfans.com (a Texas Rangers fan site)
In the past, before there was a CustomFields plugin, I've built my site around the (then) built in fields of MT. That means putting game data in the tags field, the keywords field, because it was the only place I could (then) have individual data for a specific game.
However, now that there is a Custom Fields, I was wondering this.
Is there a way to take all the data for each entry in say the keywords field and move it to a CustomField?
I would prefer to move all my custom data into CustomFields, and open up the built in fields (well pre CF, anyway) for the reasons they would have been designed for.
I imagine it would take some sort of SQL query (or a custom script), I doubt MT could do that itself.
Anyone? Bueller? Bueller?
Reported on Movable Type 4.2

I don't think MT can do that. You have to write a simple php script that would connect to the database or a plugin for MT or do it by hand. I would say by hand would be the best cuz there will be a less chance of destroying your database...
The theoretical answer to your question is, "Probably."
I'm not aware of anything that actually exists that'll do this, though. At least not publicly. It's really the sort of thing that's much more likely to have been custom-written for a client. I'd imagine there are just too many variables involved to bother trying to create a general tool, eg. what to do if someone tells it to cram a text field into a radio button custom field.