it's not going to happen, North Vietnam was ruled by two factions in the Communist Party the north-first and south-first factions.
The south-first faction knew they would lose support in the party if they did something as idiotic as reinforce the Tet Offensive with their own troops, it already happened once. The Tet Offensive was basically suicide for the NLF, even with NVA troops sure they might make a few more territorial gains but their gonna find themselves thrown back hard.
The only reason NVA troops were even fighting in 1972 is because the NLF was basically wiped out from the Tet and mini Tet Offensives.
The only problem I can see with this is that General Westmoreland would still be in command and the US would still be on that body count strategy. That's basically why the US committed lots of atrocities in South Vietnam.