Reassigning paid subscriptions to another user account is not supported.
However, you can:
First, upgrade the target subscription to a paid subscription.
Second, migrate any private feeds you have to the target subscription (ownership transfer + acceptance by target user). Migrating can be done by going to each feed's "Feed Security" tab and transferring feed ownership to a new (or existing) user:
Then, cancel the current subscription, which will be pro-rated automatically by our payment provider.
It is important that you transfer your private feeds before canceling your current subscription!
If you don't do that, these feeds will become locked out.
For Enterprise subscriptions, contact your account manager.