OSM allows you to go back in time to a previous term to see all the data for members who were with you at the time.
If members have totally left you (and not gone anywhere else), go to Members - Personal Details, select the members who have left you and click 'Remove'. You will be asked which their last full term was - they will appear in all terms up to and including this term - but they won't appear in future terms.
To transfer a member (when they move to a different sectie), use the 'Transfer' button instead. Once you transfer someone, you will not have access to their Personal Details (for data protection reasons).
If someone is going to be in two secties at the same time, use the 'Copy/Share' button so their record can be shared.