I always assumed that information about any upgrades O'Brien made to Defiant's systems were forwarded to Starfleet, so that other ships of its class could be modified. This would also to apply to Scott, LaForge, Torres, and Tucker in terms of their ships (though Torres is the exception, until contact was re-established with Alpha Quadrant).
Except that there's dialogue in "Paradise Lost" that contradicts this. Captain Benteen says, "We've been unable to stop the Defiant
. Someone has equipped her with ablative armor and neglected to inform Starfleet Command."
It's possible O'Brien eventually filed a report on the upgrades, but if he did, it was a while after they were made.