O.K., can't answer all of these but.....
ivanova, that's exactly what I was trying to do. I already have an assigned room and was hoping to grab another specific room in a different category (no, not the SPH, although that would be nice), but was told that they would only transfer to a guarantee (that's gty). I could stay in the same pricing tier, but would have to take my chances on which deck and room I ended up on. (Won't do that -- wouldn't be prudent).
There seem to be all sorts of theories as to how to get an upgrade, but from the posts here it seems that it really doesn't matter if you have a gty or assigned room. In fact, some folks have reported that they were "upgraded" from a room they wanted to a room they really didn't want. What does seem to matter is when you book. Again, just from the anecdotal reports here, the early bookers seem to get the upgrade.