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first why do you want to root?
you don't need to root to put 2.1 on your phone.
if you follow the instructions to put 2.1 on your phone. you do the same thing to put 1.5 back onto the phone. just replace the android 2.1 tarball with the android 1.5 cl14 tarball

Thanks Linc
I have saved SPH-M900_CL14_MR.tar to my backups drive in case I ever need it again.
When SDX is back up again, I will get the latest version from there and go for it!

Eazail70x7 need a computer. but you should be able to tether your phone to computer (using pdanet free) and download to computer. although you will probably have the radio issue that will stop you from downloading