Chamberlain manufactured a receiver that was backward compatible to the billion code system. It had a red learn button (maybe green). Neverthe less, you could program either the previous remotes or the security plus remotes, but you could not mix and match. thus one billion code remote and one security plus remote but not both. So you will simply need to take the remote you refer to use and purchase another one with the same remote signal.
It is confusing to many people even professionals whom are newer to the industry.