I have seen gas water heater " install kits" that use the yellow flex appliance line for the gas connection to the gas valve on the water heater. My tank was installed when the house was built and has black iron pipe all the way to the union and into the valve on the tank.
I have heard some codes do not allow the flexi line, and some require it, So my first question is what are your opinions with either option.
Im worried that when I change the tank, the current gas line will not match the exact height of the new water tank, and without being able to custom thread pipe to the right length, Using flexible line may be the best to go.
Thanks for any opinions you guys may have.

We don't have too many gas appliances in my area But flexible connections have always been used to connect the appliance to the supply. Good luck, Tom

That yellow flex is the way to go. It solves your worry and also allows for any slight movement in the heater to no affect the integrity of the gas connection. The yellow coating is also your assurance that this is the newer approved flex pipe that will not become brittle and crack like the original flex lines. Anyone who has a flex line that is not yellow coarted should remove it immediately and replace with currect code yellw pipe.

Thanks!