I was booked in an Inside Guaranty at a great rate.
About 12 weeks prior to my sailing, I received and accepted the email offer to be called for an upgrade to an OV cabin for $400.
Then 7 weeks prior to my sailing I received another email notifying me that I could expect a call with an offer to upgrade.
The following day the call came. The OV were no longer available, but was offered a balcony for an additional $200. I grabbed it. In the end, I paid the going rate for an inside cabin but enjoyed a balcony cabin instead.
If you choose the offer, you do need to pay on the spot. So, have your phone and your credit card close by. ;)