I would imagine that you can, in the past we have always allowed purchase at full price any time. you will see it here if that changes.
Additionally you should check out the Sprint Buyback Program - it can be a way to discount the full retail price of a new device by selling your old device back to Sprint! How cool is that!
something to think about: assuming you signed your contract prior to sept 9, 2011, your ETF would be less than $250 (it gets smaller the longer you are in the contract) ADDING that to the cost of starting a new contract with a different service (verizon or att) it's still less than the likely retail price of the iphone. ALSO taking into consideration that all three major networks are within $10 a month of each other, depending on your usage habits/budget that might not be a huge difference.
SO if sprint wont sell you the iphone for a good deal, you could easily switch networks and have it be cheaper than staying with sprint and paying full retail for the phone.
Of course you can pay full price (I'm sure they will accept if you want to pay more than full too ), although as someone mentioned somewhere else, if the phones have high demand and are hard to come by, Sprint is going to provide phones for new contracts first. Even though they will make more money on your phone sale, they will make even more locking someone in for two years.
You are pretty much going to waste your upgrade discount when it becomes available... or at least postpone realizing it's value until you actually buy another phone, assuming you're willing to go two more years at that point. I'm pretty much in the same boat as you, and coincidentally, my EVO has been acting up of late, too. Hard resets seem to be the only fix. Will get tired of reloading my contacts, apps, and all those personal accounts and settings.
Looking into the option of paying the ETF at this point might make sense.
The preorder is currently only for the iPhone 4, which is only available in 8GB. The iPhone 4s will be available for pre-order tomorrow (Oct. 7) at 12:01am PST in 16GB, 32GB, and 64GB configurations.