The only answer I can give is that it should upgrade OK. This sort of question is always difficult because you may have done something unique or work in a unique environment.
I would install the trial version on a non production machine and upgrade a copy of your project to test. Do not upgrade the original until you are ready to go ahead.
Plenty of help here if you should hit problems.
See www.grainge.org for RoboHelp and Authoring tips
In general, as Peter said, you should have no problem and everything should import properly. I have had some instances where files with extremely long names don't convert well. So this is something you might want to check.
In general, it is highly recommended that you first copy your file over to a new location before importing it - and if you are concerned, run a demo version on a smaller project or portion of a project just to see the results.
From my experiences, conditional tags, TOC, index, links, etc. - all come in fine. CSS etc. work well...no major showstoppers.
What if someone else did the conversion and didnt save a backup in rh9 first .....they just went ahead and did the conversion so now the project is in rh11 but the boss wants to keep the source files! Or the rh9 version incase there are production problems with the rh11 version..is the rh9 version just gone or overwritten in this case?
In that case you would need to "reverse engineer" the files or manipulate them so they could still be used in the earlier version.
You might use some scripts you obtain from Willam's site (click here to visit)
Or you might use the information you obtain from Peter's site (click here to visit)
The RH9 version is updated to RH11 so project files and topics are changed. You can reverse engineer from an output as Rick says, you can also downgrade your project back to RH9 (albeit with some limitations). http://www.wvanweelden.eu/articles/downgrade-robohelp-project