What I read is that the manufacturer's manual can establish a service-life limit for a parachute, but if it is published as a replacement for an existing manual issued for a pre-dated parachute, then the manufacturer must file an SB.

Regardless, it seems a real stretch to think a rigger is compelled to repack a chute even only 5 years old, let alone one 20+ years. I assume they decide what risk they want to take, as long as it doesn't violate the regulations.