Water temperature in upper shower
Single handle shower valves normally have some way of adjusting the "all the way hot" stop of the handle.  In many cases it's just a toothed plastic ring under the handle that can be moved to the next tooth or two to make the water hotter.  This is meant to be a safety feature so even if the handle is cranked all the way on you don't get scalding hot water.

Your new water heater might not be set for quite as high of a temperature as the old one, so you're two choices are to adjust the shower or adjust the water heater.  Note that adjusting the heater temp might make all the two-handler faucets hotter, so consider that.