That's not a consideration here. They were just replacing a smaller 100A panel with a larger one. with more circuit spots. That is always allowed. As long as y

That's not a consideration here. They were just replacing a smaller 100A panel with a larger one. with more circuit spots. That is always allowed. As long as you you use a panel that accepts the same type of breakers, you can also re-use the existing breakers, unless code require upgrading to AFCI breakers. Otherwise you swap them for new breakers of the same rating. (Assuming the US: There are some inspectors who would ok a panel swap that reused the existing breakers without requiring upgrades, but technically upgrading is supposed to be required, unless your state amended out the AFCI requirements in the state version of the code). Note: technically many breakers can fit in competitor panels, but they are not legal. They were not designed nor tested to trip at proper ratings in those panels, and using them violate the labeling of the panel. Now obviously the reason for upgrading the panel is to add more circuits, and it is possible that these additional circuits if added would need more power than the 100A service can provide. To technically comply with code, you need to follow the rules to calculate the expected loading of existing circuits plus planned new circuits.