I usually don't recommend using the drop down box that let's you switch between object types, if the wrong XML get's left over things like this can happen. That's why it's happening, because of the XML that's left over after you switch the field type. I get the same thing if I try it in 7.0. Seems like this was fixed in 7.1.
Anyways, you can fix it the hard way and manually edit the XML. Or the easy way and just delete the field and replace it with a new text field. Unless you have a really good reason I'd suggest the easy way.
Chris
Adobe Enterprise Developer Support