This SQL query will clear the expiration date for active recurring memberships in Paid Memberships Pro.

set_enddate_null.sql

# BACKUP FIRST
# You generally do not want to setup PMPro levels
# with both a recurring billing amount AND
# an expiration date.
# This SQL query will clear the expiration date
# for active recurring memberships.
# BACKUP FIRST
UPDATE wp_pmpro_memberships_users
	SET enddate = '0000-00-00 00:00:00'
WHERE status = 'active'
	AND cycle_number > 0
	AND (enddate IS NULL OR enddate <> '0000-00-00 00:00:00')