Yes...it should. We have just completed the upgrade from 8.5 to 9 and haven't had any problems. That being said, it is possible. I would suggest making a backup copy of all of your apps before you start using them in 9 though. If you have backups, I wouldn't worry too much about it.
Just a note of caution here, because we just finished the upgrade on our UAT environment and suffered quite a few problems that appear to be very specific to our documents, I have no confirmation from QlikTech yet, but I am under the impression that the SR4 plugin client and all thick clients are calculating every object on the sheet even if its currently hidden. This has a huge impact on our performance because our apps only ever contain one sheet, one of our production apps has well over 100 chart objects on that sheet with only approx 6 visible at any one time.
V9.0 SR4 on the Server with a V9.0 SR4 client runs like a complete dog, it is unusable!
However, V9.0 SR4 on the Server with V9.0 SR3 as the client runs perfectly!
As I said, this is very specific to our apps, but if you suffer performance issues when upgrading, bear this in mind.