my Hot Water branch circuits are 1) 1/2 to the kitchen
As long as that pipe supplies nothing else besides the kitchen sink and dishwasher, you are fine to tie that existing 1/2 into your new upgraded mainline.
2) 1/2 to the two upstairs bathrooms
This needs to be upgraded all the way up.
3) 1/2 to first floor lavette and
What's a lavette, is that like a powder room or half bath (no shower or tub)? A powder room only has one lav sink that requires hot water, so you could tie the existing 1/2 into your new upgraded mainline.
4) 1/2 to washing machine,
As long as that pipe supplies nothing but the washing machine, you are fine to tie the existing 1/2 into your new upgraded mainline.
Me thinks I will upgrade the existing 3/4 coming off the meter to 1 inch,then change all the 1/2 water supply to the Hot Water Heater and 1/2 hot water distribution line to 3/4.
This will give me more than enough pressure and flow for my system,based solely on my own knowledge of this house.
As to why the plumbing code doesn't address this exact sizing situation?????