Registered subdomains (SLDs) can always be renewed at the locked in prices from the time of the original registration. When a subdomain is first registered the renewal prices are fixed in the registration contract and renewals will not be more expensive than the original price. This is to prevent TLD owners increasing registration costs and pricing out existing holders.
TLD owners can apply discounts to renewal of domains.
A subdomain can be renewed by any address, not just the subdomain owner and approved addresses.
Minimum renewal is 1 day, maximum renewal time is 100 years.
Renewal discounts are applied from the person that is submitting the transaction.
There is a 30-day grace period for SLDs after they have expired. During this time, the SLD cannot be purchased on the secondary market, and it also cannot be transferred. The SLD will not resolve through the public default resolver during the grace period.
Any days passed in the grace period will also have to be paid for if the domain is extended. This period exists to protect holders from losing their SLD due to a missed renewal.