Water pressure & supply line question

Hello! I'm adding a shower and the fixture recommends 3/4" supply lines. The city line comes in at 3/4" and then it is reduced before the water heater. If I change it to 3/4" all the way up to the bathroom, can I "T"-off to the existing 1/2" lines to the kitchen, 1st floor bathroom, laundry, etc. How would this effect the water pressure throughout the house?

Does it reduce to 1/2"?

You can repipe to 3/4". That will greatly help the flow although many meters are 5/8" inside. How old is the house?