Perhaps an agency had a backdoor and then changed the lock to let another agency in without giving access to prior intercepts or even acknowledging that they ha

Perhaps an agency had a backdoor and then changed the lock to let another agency in without giving access to prior intercepts or even acknowledging that they had prior access? Say NSA had a backdoor and then replaced it with one shared with GCHQ. In that scenario, the NSA has not lost any capability while GCHQ has gained capability and NSA has plausible deniability that they did have prior capability.