I just bought a used Flex 6400M

Nothing to do about change of ownership. To operate remote, a new smartlink account will need to be set up.

Hi KA6AKH...welcome to the Flex Community! Drop by anytime, we're glad to help.

By the way, the 'licence' as to which version of SmartSDR you can use goes with the radio, so if the radio has version 3.xx then you are good to go. If it has version 2.x and you want to make use of 'multi-Flex' (where two people can be logged onto the radio at the same time), then you'll need to upgrade the radio's licence to v3. It think the cost is still $200 USD. Check with the help desk, or sales @ Flex to confirm.