Nancy and I do guarantees on a regular basis, plus we've had hundreds of our clients book guarantees. The overwhelming majority of the time, you get upgraded when booking a guarantee. However, most of the time it's an upgrade with the same type of category, i.e. inside to inside, oceanview to oceanview, etc. While upgrading from one type of category to another (inside to oceanview, etc.) does happen, it's not as common.
As I always tell people, booking a guarantee is a gamble; sometimes you win, sometimes you lose. You could end up getting a nice cabin for a nice price, or you can also get a terrible cabin for that same price. We've gotten an inside cabin as far forward on the ship as you can get. If you're prone to seasickness, this is not the cabin you want. It didn't bother us, but there was one night we were literally coming up off the bed the seas were so rough. It was a wild ride that night - we actually enjoyed it, but others around us didn't fare so well. The other bad thing about the cabin, it was right below the stage in the main show lounge. We could hear them dancing around up there while practicing during the day and for the shows at night.
We also ended up below the Lido Deck on a 14-night cruise underneath the pool area where there was a wooden floor. It was non-stop noise above us for 2 weeks and drove us nuts!!
But the good thing is about a guarantee is that often the price can be alot cheaper.
Also, you really need to book with an agent and not through those large online sites. You want someone who will look after you. And with guarantees, it's important to have a good agent because sometimes if you're assigned a cabin that's not quite so good, they can work their magic to get things changed. Not always, but they do have contacts with the cruise lines not available to you and can get some help when needed. And sometimes they can even get a bit more for you. The people working for these large sites aren't travel agents, don't have the knowledge, nor do they want to spend the time researching.