Most resorts have a rule that you can only reserve what you own. That rule is there for a very good reason. If a resort routinely allowed you to reserve "up" to a bigger unit, why would anyone buying from the developer (someone has to do it!) consider buying the larger more expensive unit?
Also, since every week is deeded in most timeshares, you have ownership in a 1BR or 2BR week. Imagine the outcry if 2BR owners tried to make reservations and were told, "Sorry, we gave the rest of the 2BR weeks for this year (or for that week) to some 1BR owners!"
Thus, if you buy a week at a resort that has multiple-sized units, you should develop a mindset that the size you buy is what you'll get. Period. It doesn't hurt to ask when reserving or when checking in during low season, but it will be a rare situation when you can upgrade.
As for banking a week with an exchange company, you can only bank what the resort has. Thus, if (in my earlier example) all of the weeks for the second half of 2007 have been reserved when you call in July, the resort won't have any weeks to bank for you. Remember that when you deposit a week with an exchange company, someone else will occupy that week at your resort. So if the remaining 2007 weeks have all been reserved by others, you will just be out of luck for 2007. The one exception is that some resorts do bulk week space banking under which they deposit a number of weeks with an exchange company in advance and assign one of those weeks to you when you indicate you want to deposit a week.
Bottom line: With a floating week resort, make your tentative plans at least a year in advance and make your reservation as soon as reservations open for the week you want. By doing that, you'll have numerous options available if the week you want has already been completely reserved by others. And if you later want to change your plans, you can ask the resort to change your reservation (possibly for a nominal fee) if the new week you want is available.
Some people suggest that floating weeks won’t work for them because they can’t make plans that far in advance. Hah! If you own a fixed week, you are making plans years in advance by buying that fixed week! If needed, there are always options for changing the date of a floating week reservation – change the reservation date, exchange with an exchange company, rent the week and use the proceeds to rent the week you now want, etc.