Sorry, I lied.
Here's the enhancement table for the sword:
Success rate is 30%, break rate is 25%, and if it doesn't succeed or break (45%), it can delevel anywhere between 0 to 2 levels. So yeah, there's a chance of the sword staying at +9, but there's also a chance of it going back to +7.
What's interesting is that apparently you can lose a level while going from +3 to +4. Gotta fix that page on the wiki I guess. If we can't find a pattern per item rarity, I'll just include the success/fail/delevel stats on each item page (like DNArmory has).