John’s baptism

The charge that if John’s baptism was not Christian, then all the people John baptized would then need to be actually baptized for the first time is flawed in that all of these people already had the sign in their flesh already-unless of course you are prepared to say that the sign of the OT was less than efficacious and now invalid. If so, where are the mass baptisms in the NT transitioning all believing Jews? yes, we have Pentecost as an example, but thats just 3k souls who agreed to submit, given the event. But there has to be a transition period. A grandfathering in of those who had the sign already.