DG makes a 7-7-7 which I think could be used from seed to harvest. You may not be getting your best yields off that and running really green plants at the end, but if you used that for the first 30 days and used bloom (3-12-6) for the last 30, you'd be in great shape. Personally, I run 1ml of grow and 6mls of bloom for weeks 5 and 6, then switch to bloom only for roughly the last 2-3 weeks. The 3-12-6 will slowly turn your plants light-green with some yellow leaves here and there and initiate a slightly higher rate of leaf-drop which isn't necessarily a bad thing during the last few weeks. I'm still tweaking my own schedule but this is where I'm at thus far.
In regards to not switching out water, I think some people get away with doing that with good results. Personally, I'd change the water with fresh nutes once per week. You can add back nutes each week using a TDS meter to gauge how much food you should add back, but you're not starting with a clean slate which I think could get a little confusing as the nutritional needs of the plant change throughout flower. Switching out water and food once per week is worth the effort in my opinion. Then again, you're talking about DWC which is a different animal to manage than flood and drain.
. Additionally, I only feed my DG dirt girl 3mls of bloom per watering. She's doing very well and at 3mls/gallon, that plant will cost about 50 cents to grow.
