Check here http://files.pspupdates.qj.net/cgi-bin/cfiles.cgi, in the upper right corner, the 3rd most downloaded I believe is the 1.5 downgrader.

As for upgrading again I think you just have to put the 2.0 eboot (which you can find on the same site) in the update folder, or use an in game update like off of GTA.

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