But you have to renew your license. It’s not “buying them again”, but when you make changes to the software suite, all parts of the suite need to be up to date. You can’t add the current Pages to an old install, that hasn’t been updated in 3 years and you also cannot get the 3-year-old Pages version that would fit the old installation.
Updating, at least from time to time, is highly recommended anyway. You can have your license lapse for some time if you have too, but the longer that lasts the more problems you get, because security holes aren’t closed, bugs aren’t fixed, third-party connections aren’t updated and so on.