How can I rename an auto-named constraint when I get this error 15248? Method:

Open SSMS. Find table. Open constraints. Find constraint name. Right click, rename. Give new name.

EXPLODE. (well, it just fails, at least)
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';