In ArcGIS Online, set a fixed start and end date for a members named user and/or role OR no dates ( infinite). You also set the next course of action if the end date passes ( its mandatory). These are
1) revert the member to the new member defaults" set in Portal, OR
2) disable the membership, OR
3) rever the member to a custom member type and/or role (set this here)
14 days before then end date, a message\email is sent to user and admin to take action. Those actions are to 4) redefine the dates again or 5) make the dates infinate (no dates set) or 6) revert member to a new membership type and/or role or 7) disable membership).
After the end date has passed with no action made, an action occurs automatically based on what was set at the start.
Any named users that are reliquished are put back into the pool. For action 1 or 3, there always needs to be +2 named user types in the organisation pool available. If there isnt, then a warning message is provided at the start and users will need to choose a different action point.