I didn't wipe my phone or remove my media card or any of that. Basically, I would back up the phone, uninstall the current version of OS from your p-c, download the new version to your p-c, delete the vendor file and install it to your phone. (You shouldn't have any problem with DM recognizing and installing a new OS.)
Be patient. It only to me 30 minutes but some have taken as much as 1 1 /2 hours I think. Sometimes it looks like nothing is happening but leave it alone and let it run!
Reserve a little time for testing/installing any 3d party applications. For example, if you use BB Smart for email, don't install if for 4.5. You may have to try a new theme. The T-Mobile theme that I was using omitted my BB messenger icon. I changed to Dimension L. You may have some lost contacts but they can be restored. All that info is available - that's why I recommended reading up on the upgrade in the thread I referred to previously.
You should be fine. Good luck and post back results here or in one of the 4.5 threads for help.