Here are the instructions in the FAQ: How do I downgrade the OS on my BlackBerry? - BlackBerryFAQ
However, the new Desktop Manager does not downgrade automatically, you have to tell it to install the old OS and then it'll warn you that it is an older version. To which you just approve that.
You generally do not uninstall the OS, it is closer to overwriting it with the other version. To uninstall it, you would use JL_Cmder to wipe the device then install the OS. However, you don't generally have to wipe the device, you can just install the OS you want and Desktop Manager will remove the old OS itself as part of the install process.
Before you do that, I'm not sure what the roaming only feature is, but are you sure the new OS doesn't have it or it can't be activated again?