That is what the callsign effective date range is for in the My Profile page (
http://www.eqsl.cc/qslcard/editProfile.cfm)For an active callsign you set the start date to be the date you received the callsign, and then set the end date to some date in the future (2020 for example)
For an old callsign you would set the date range to be the range you had the callsign. That way the current owner (or previous owner) of the callsign can also register an account with the same callsign.
73, Dave - VA3KAB