My wife and I are remodeling our bathroom.
One part is to replace the fiberglass tube/shower surround with a 60X34 tile spa shower with 4 body sprays, a hand held shower and a rain shower head.
The fixtures we are using are by Moen and require 3/4" supply lines.

My problem is the cold water supply is 3/4" through the house but is T 'ed off to 1/2" at every fixture in the house, Kitchen, Bathroom sink, and Tub/shower.

The hot water supply line is 1/2" from the furnace to every fixture.

I think I can replace the T reducer with a 3/4" T for the shower cold supply.

But, can I install a fitting to go from 1/2" up to 3/4". If I can will that cause me any other problems? Or do I have to install all new 3/4" pipe?