hello! Not an expert but I just wanted to share that in my experience upgrading from CF MX 6.1 to CF 11, I didn't really have any problems. All the webservices I was running in 6.1 ran smoothly in CF11.
You will only be able to purchase a CF11 license. You wont be able to get an upgrade to it from 6.1. The versions in between are all old and unsupported now so you wont be able to get licences for them either.
There is a function in the CF Administrator called Code Analyser that you can use to run your pages through to check for code that has been depreciated or may cause issues. SQL queries may also be an issue if its that old.
I would recommend getting a trial of CF11 and checking your code in the Code Analyser. There will probably be a few things that need changing, but its something that will have to be dealt with unfortunately.
MMany thanks to both of you that answered. The code analyzer will be a tool we could use as we will be cloning the VM's that run the CF code and can upgrade them, use the CA, etc.
We are woefully behind in upgrading but your advice is great and appreciated.