I had the 7105t and already went back and forth a couple time on OS without any problem, this is how I do it. (tmobile4.0>csl4.1>csl4.0chinese>csl4.1)
Download desktop and handheld 4.1 from the Hong Kong CSL site.
https://www.blackberry.com/Downloads...B42B2DBB8F27B6 (Not sure 7290 is same as 7280)
Uninstall both desktop manager and handheld software from you computer. (you should have only two blackberry entry under windows ADD AND REMOVE PROGRAMS)
Use java loader and wipe handheld.
Install new desktop manager and handheld OS software on the computer.
Locate vendor.xml, either modify or delete it.
Hook up computer to handheld and start application loader.
EDIT-- Did you install handheld OS onto the computer, I didn't see you mention that on your post.