first off, do a backup BEFORE doing anything else. so you have your contacts and messages and what not. I ALWAYS USE JLCOMMANDER for a complete wipeout before installing. i never do the upgrades as most times i have problems.
i have also noticed then when you download the file. it does a install method on the pc. before it allows you to install to phone. so if you had 5.0 installed. then you want to go back to 4.5, persay, 5.0 gets reinstalled. to avoid this problem. you go into your control panel. add/remove programes. and remove the 5.0 installation. reboot the computer then open up the appropriate file you want to install. delete the vendor file , plug in the phone and click the apploader file and you will be golden.