Website admins can give a person complimentary Pro from their page in Admin: for 1, 3, 6 or 12 months, until a chosen day, or with no end date. It gives the same limits and areas as paid Pro. Admins can change its end date, end it early, keep a private note, and email the person about it. Paying customers lists everyone with complimentary Pro.
Complimentary Pro and a paid subscription do not change each other. A subscription keeps renewing, and cancelling it does not end complimentary Pro. When complimentary Pro has an end date, the billing page offers Keep Pro after that day: a plan chosen there takes its first payment when complimentary Pro ends. The billing page and the mobile app show complimentary Pro with its last day.