3/4 gas line reduced at T to 1/2?

I have 3/4 gas line that branches with a T (3/4x3/4x1/2 to the appliance). I am installing a tankless water heater that specifies a 3/4 in line. Can I use a 1/2 to 3/4 adapter at the T to increase the line size or must I replace the T to 3/4x3/4x3/4? Thanks