Theoretically you should be fine, you can certainly go 9.1 to 10.0 inplace.
Actually getting it to upgrade correctly is another thing, I've found that more often then not it'd have been quicker to just uninstall (often manually) 8.6 & 9.x and install 10.x from there, I've found their installer routines are a bit shoddy and tend to screw up the upgrade. Probably about 4 of our 7 9.1->10.0 upgrades didn't work. Fortunately we didn't have tooooo much customisation to lose.
Not saying yours will have problems, but be prepared to start from scratch as you never know.
