[renhanxue]

The Voyager software was updated repeatedly and significantly in flight while the spacecraft were still in their early years. They were very intentionally designed to be patched over the course of the mission. The software as launched was not capable of a meaningful mission beyond Saturn, because for budget reasons that was officially not on the cards at launch (the Voyager name came very late; the program was officially "Mariner Jupiter-Saturn" for a long time). Features like image compression were added in the early 1980's, after Voyager 2's mission extension to Uranus and Neptune had been approved.

Without any inside information on the program, I would expect that a lot of development has been done more or less ad-hoc over the decades, as budgets have allowed and operational requirements demanded.