I just alternate on topups. I'll use straight G12 once and then water the next time. I keep both in my trunk. Unless you were filling up from dead empty, filling the reservoir with some straight G12 isn't a big issue. Draining isn't really necessary, IMO.
Also, I think, depending on where you live, weather, etc. You can run up to a 70/30 mix coolant/water. I really wouldn't worry about it.
Mine: 20th AE GTI, BMP, 123k, stock...for now
His: '01 Jetta TDI, 122k, auto -> manual swap
The project: '84 Jetta GL TD, Giles IP, 2.5" downpipe and exhaust, full rebuild done winter 2010/2011