We got our Interior Guarantee Cabin 30 days before our Brilliance B2B for the first portion on Deck 3. On the 2nd portion we got upgraded to a Balcony on Deck 8. We are still holding out but are not expecting a RoyalUP upgrade. What Royal told me is that they will do everything to assign our cabin before boarding but there is a slight chance it would happen at the port.
A guarantee cabin is sort of like Basic Economy on the airlines you pay the low cost and if you don't want to pay for an assigned seat you get the luck of the draw. Now what I think happens is Royal may look at your Crown and Anchor Status, Past Sailings and assign you to the best location possible.
We are very happy with our Interior location just forward of the forward elevators at the end with no cabin on one side of our cabin as its as the start of the block.
Some may not be happy with a Guarantee and I suggest that they pay to get an assignment but for those who are not picky then the savings can be grand!
Hope this helps.