Hello Maria Victoria,
JFS license model is in general just the same as the license model used by Atlassian for Jira Server. The license does not expire, but support & maintenance period expires. Support & Maintenance renewal policy is ruled by the clause 7 of the JFS EULA: Subsequent Renewal Periods commence upon the expiration of the prior Software Maintenance regardless of when it is purchased. This means that upon purchasing of, say, 1-year renewal, this 1 year will be added to the current expiration date of your support & maintenance period and not to the current calendar date. You may find the similar statement for Atlassian products here.
In this particular case support & maintenance period expired more than a year ago, so it is just cheaper to purchase a new license than to renew an existing one to reactivate its support & maintenance in order to use the most recent JFS versions, and that was my recommendation.
However I sincerely apologize for so delayed reply to your support request. I definitely should not have kept waiting for so long.
With best regards,
Alexander