BP Follow currently doesn't have a tool to convert existing members over to followers.

However, community member @jwenerd provided a script to do this:

https://gist.github.com/jwenerd/71d22393a4d377fbfd21

Note: This script is not supported officially.

Leave questions and feedback about the script here: https://github.com/r-a-y/buddypress-followers/issues/21