The Bangkok Post reports that Don Muang will be officially reopening this weekend. Initially both Nok Air and Orient Thai Airlines will be using it for all their flights while THAI will be using it for their non-connecting domestic flights only. On all new ticketing the code for Don Muang will be changed from BKK to DMK.
According to the Thai Airways site, flight operations at Don Muang Airport will be numbered 4 digits starting with 1 (TG 1xxx) such as TG 1203 and those operated at Suvarnabhumi Airport will be numbered 3 digits. They also have some timetable information here.