If today's date is after a member's expiration date, then the member will be considered Expired unless the member is subject to your Grace Period. Your business rules might call for expired members to be converted to Prospects at some point in time.
Prospects are records that have a member type but have no "Member Since" or "Membership Expires" dates. They are also not entitled to member benefits.
In Novi, this is a semi-automated process that can be done on the record level or in batch.
Warning: Converting a record to a Prospect will remove the membership since and expiration dates, which will affect reporting and historical data.
On the member record, click on the "Details" tab. Delete the dates listed under "Current Membership Since" and "Membership Expires." Save the record.
To convert multiple members to prospects, find the records on the member list view (/admin/customers/customer-list). To find expired members quickly, consider using the filter.
Select the records and perform a "Batch Action" to "Convert to Prospect."