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 is done by going to each feed's Feed Security tab, and transferring feed ownership to a new (or existing) user:
Finally, cancel the current subscription, which is automatically pro-rated by our payment provider.
WARNING!: It is important that you transfer your private feeds before canceling your current subscription!
If you don't do that, these feeds are locked out.
For Enterprise subscriptions, contact your account manager.