Quoting Orkie from a previous thread
"Correct, flashing once probably won't cause problems but there is no guarantee of this. I had a totally dead GP2X after about 10 flashes (to give an indication of how little it takes sometimes), so flashing a few times probably won't cause trouble it is much better to avoid that wherever possible. It is not a 'quick fix' with JTAG either, it took me weeks to come up with a solution and about 5 hours to pull it off. It is easier now that I've done the initial work with the fixing tool, but this isn't publicly available and even if you did have it, it can be a struggle to get it loading sometimes over JTAG (serial may be more reliable, but if you happen to have destroyed the bootloader too then there is no way of doing that). I assume writing to the NAND normally will also cause these problems, but I've never tested.
This is a matter that needs to be taken seriously. Most people don't have the equipment or time/knowledge to repair it. That said, if something doesn't work for you or there are new features in a newer firmware, this shouldn't put you off upgrading as it is repairable. It's just too much bother for a downgrade with no real benefit"