Can’t DIY here, must have the proper licenses and permits to install..............
I can understand this to a point...afterall, if you replumb the gas and water lines one can create a lot of problems if not done correctly.
I’d settle for permits and an inspection, but the license thing goes a bit too far in my opinion.
Changing to tankless isn’t just a swap of the heater. A licensed plumber will know that the vent on a tankless often has to be about 2.5 times the capacity of the vent on the plain gas fired appliance it replaces.
Vented gas flue is hotter and higher capacity — let’s not have anyone burn their house down.