Can I revert back to Ubuntu 9.04 when I've upgraded to 9.10? I really want to install Ubuntu 9.10 but I'm scared if it does not work, if I will be able to go back to 9.04?
Not really. There is a mechanism in apt for downgrading packages called 'pinning', which could theoretically be used, but downgrades in general aren't officially supported or particularly well-tested. So if you're worried about getting problems while upgrading, you should be even more concerned if you want to downgrade.
The best four ideas if you want to keep a stable system:
(1) wait. A fair number of upgrades come through the pike in the first few weeks after an Ubuntu release, since suddenly the number of installations and the variety of hardware and application use goes up radically. You'll have better odds of upgrading without trouble if you give it a month, even two. Also by then any problems that won't be fixed in this release cycle will be better documented and may have unofficial workarounds.
(2) Install a trial installation. Can you make room on your hard drive for a small fresh install? (Give it about 5GB, for the base and your most crucial apps). Play with it for a while and see how it goes. If it works, backup and upgrade your old system. Then you can see which one works better (upgraded or fresh). I always do this, since it lets me see how things work on my hardware without any disruption. It's pretty safe to resize partitions these days (but do backup, and if it's NTFS, defragment first).
(3) Try the live CD. It's not the real deal, but it may give you an idea of the things you won't have problems with.
(4) You can of course backup your whole partition with something like PartImage before you upgrade. This will also give you a path to replace your new install with your old one.
But really, just don't go for the bleeding edge. You value the stability of your system more than that.
I suggest installing the Karmic 9.10 on a 4GB USB drive to try it out.
Why use a USB installed Ubuntu,