Reference Book: Star Wars Saga Edition Starships of the Galaxy
See also: CR70 Corellian Corvette, CR90 Corellian Corvette
When the Corellian Engineering Corporation began producing the CR90 Corellian Corvette as it's standard (Rather than as an upgrade), current owners of CR70 Corellian Corvettes were given the option of purchasing an aftermarket modification suite that brought the vessels up to date technologically with the CR90 Corvette. Known as the Vanguard c20 Retrofit, this modification enhanced the combat prowess of the Corellian Corvette and ensured that the CR70 remained on par with the CR90, essentially transforming the former into the latter. The Tantive IV, for example, began as a CR70 Corellian Corvette that was used by Bail Organa at the tail end of the Clone Wars. By the time of the Galactic Civil War, the Tantive IV had undergone retrofitting that transformed it into a CR90 Corellian Corvette.
A Vanguard c20 Retrofit costs 500,000 credits (Which includes both raw materials as well as labor costs for having the retrofit applied) and usually takes 2 to 3 weeks of work by trained professionals. The process changes the base statistics for the CR70 Corellian Corvette to those of the CR90 Corellian Corvette, though this does not change the Crew Quality by default.