Not only is it complicated but you are also required to have the same version of each extension on both sites, and I couldn't find a lot of the versions that were listed in their documentation to download for 1.5, they are simply too outdated. In addition, not every extension is support, even if they are available for both versions.
Not even the users would update on 2 of the sites, and yes I had the settings setup correctly. The few extensions in which I could get on both sites also didn't migrate.
It was a waste of my time and a waste of my money. I would not recommend going this route for upgrades.
I am giving a 'Fair' rating as their support team is fast to answer emails.