I noticed this now a couple of time so i thought i would report this as a possible bug.
When you have an existing field of the type Text, Textarea (possibly also Datetime, Email and URL) and trying to change the type to Concat it triggers a error message.
I have tried this with a field of the type Textarea, then the Type pulldown has the following options:
Concat
Datatime
Email
Text
Textarea
URL
After the error is shown i have tried again to modify the type from Textarea to Concat and that worked fine, no error.
It looks like it has problem trying to modify a field that needs no actualy table in the database (concat field)