This is all explain on our site here. One owned license grants you the right to run one software installation on one domain for an unlimited time, and purchasing an owned license gives you access to our members download area and support for one year. After that year yes you need to purchase a members area renewal if you want updates or support. Those do not continue forever. Your license is good forever meaning if the last version you had was 6.2 you can run that for 75yrs and never pay another dime.
To upgrade from 7.03 you simply upload the changed files. There is a complete change log in the documentations folder of the download.