The expiration date on the solid rocket booster wasn't actually for the boosters themselves, but assembled product. The clock didn't start with the manufacturing of the booster segments, but when they were [combined together][1] near the launch pad. 

I can't find a source, but I believe the timer is for the o-rings and connecting hardware. The concern is that the same thing could happen that happened with the Challenger, the seal breaks mid-flight and causes fire to come out the wrong area, leading to the same issues that happened with the Challenger breakup. 

If there had been a determination that this timer was expired it would have required disassembling the segments of the solid rockets and rebuilding with fresh connecting hardware. The same segments could have been used with the same fuel. 


  [1]: https://spaceflightnow.com/2021/01/15/nasa-continues-stacking-boosters-for-first-sls-test-flight/