Thanks sic0048! In fact I did that as I hadn't got any advice - ignorance is bliss! Just for the sake of any others who, like me, aren't experts and may not have the time to do all the research necessary: you can update to 5.0.1, I backed up using PKGBackup first and then immediately jailbroke. I found I was missing a very useful app (MxTube) and I was able to go the PKG backup and just select that to restore! All in all, a very successful and rapid transformation!