Our mail setup is as follows:

~/mail/ - This contains single mailboxes (Drafts, Trash, and others)
~/mail-Archives/ - This contains a hierarchy of folders containing 

mailboxes

For example, a tree of ~/mail/ would look like this:
   ~/mail/
   ~/mail/Trash
   ~/mail/Drafts
   ~/mail/sent-mail
   ~/mail/My Stuff
   ~/mail/Important
   ~/mail/test/SomeFolder  <-- this would never happen, but I have to
                               account for it during our transition

A tree of ~/mail-Archives/ would look like this:
   ~mail-Archives/2009/MyStuff/MyStuff-Dec09
   ~mail-Archives/2009/MyStuff/MyStuff-Nov09
   ~mail-Archives/2009/MyStuff/MyStuff-Oct09
   ~mail-Archives/2010/MyStuff/MyStuff-Jan10
   ~mail-Archives/2010/Important/Important-Jan10

All of that works, I can open up Thunderbird and browser/read/delete 

messages no problem. However, when I try to move that 'test' folder from ~/mail/ into ~/mail-Archives I get an error message:

   'Can't rename mailbox to another storage type.'

The same thing happens if I try to move any of the mailboxes in 

~/mail/ to ~/mail-Archives/ . Is there a way to fix that? Our namespaces look like this:

namespace private { separator = / prefix = "mail/" location = mbox:~/mail:INBOX=/var/mail/%u inbox = yes hidden = no list = yes # for v1.1+ }

namespace private { separator = / prefix = "mail-Archives/" location = mbox:~/mail-Archives inbox = no hidden = no list = yes subscriptions = yes }