if a sim is well built, then it follows RL processes. In this case, the tire model is (should be) independent of the other physics aspects (chassis and aerodynamics and also suspensions). If it is well built, then it is (as DK claims) modular, which means you can take out the OTM and put in the NTM.
I can understand rolling out the NTM in phases (the initial release was more of a marketing necessity, especially considering they didn't have the people to a roll out across the board), which is why you have to maintain two entirely different TMs in the same engine (what a mess that can be).
Problem is: what if the NTM is not exactly legacy-code free? That is the only way to force the coexistence of two entirely different pieces of systems of physics algorithms. Which means, the NTM holds inside the OTM, which can be switched on or off depending on the roll out map.