How can I rename an auto-named constraint when I get this error 15248?
You can use sp_rename manually.

Note that this is fixed in SSMS 2012 (I don't remember if I have SP1 installed or not).

EXEC sp_rename
    @objname = N'[myschema].[DF__AccountTy__Allow__7E3AEC52]',
    @newname = N'DF_whatever',
    @objtype = 'OBJECT';