Your understanding is a bit flawed -- users don't typically get promoted from one group into another, they have groups added.
Every registered user should always be a member of the Registered group. Then they get additional privileges based on any other user group they get promoted into. So your verified users will be in the Registered group and
the Verified group. If they pay for a membership, they get added to whatever you name your paid group, and get additional privileges from that group. When their paid membership expires, they are removed from the Paid group, but stay in the Verified and Registered group. Paid memberships will not conflict with the Registered/Verified promotion.
Read the entire manual section on User Groups and Promotions
-- this should make it a bit more clear.
XenForo Manual said:
All users must belong to at least one user group but can be members of many. We recommend that all users have the registered group as their primary user group -- including your moderators and administrators! When a new user registers, they will always be put in the registered group automatically.