From my experience with upgrade from CMOD, you can go from CMOD V8.3 -> V9.5 without any stop anywhere. I've been there and done it.
You can go basically from V8.5 -> V10.1 without any problems too. (haven't tested it, but I am sure that from V8.3 -> V10.1 should be ok without problems too).
Of course if you have RDF... then you must stop at V9.0 first, like you've experienced.
Having worked with IBM, I can say that some IBMers are morons when it comes to CMOD, they think they know the beast, when in fact they know nothing. So when you hear one of them that you need to do a stop to every version, then you know that he doesn't know what he is talking about...
There was a need to stop from at version 7.1.2.8 because of a change of users, and it was clearly explain in the upgrade documentation.
Now if you read the Upgrade path to CMOD V10.1 (
http://www-01.ibm.com/support/docview.wss?uid=swg21446135) you can go basically from V9.0 to V10.1.
And having tested, you can go from V8.5 -> V10.1 without any problems...
Now what is a problem, and could cause a LOT of headache, is not the server part, but the client part.
Depending on what you have, you might need to recode part of your java program, deploy the CMOD Client, you are still with eClient / WEBi / arswww.cgi... and because of that you need to be extra careful with the version of the server. And the Client / Server Matrix is your friend to know where to go, and when to stop...
I had a customer, where I could not upgrade to CMOD V9.5, because they had a lot of legacy code, and the highest I could go with them was CMOD Server V9, with ODWEK V8.5, and I was using WEBi, because they still had IE V6...
So the problem of the upgrade is not CMOD itself... but the whole infra around it that will dictate what you can or cannot do.