you're good there, no matter the time, as long as the cats are damaged or clogged they can be replaced.
that is the only time it's actually legal to change them out.
To make it legal however, the new cats must mount in the same location as the previous cats.
as far as passing a sniff test, I'd find someone with an aftermarked catted h or x...and ask them how they passed...there is always a way!