GE needed to retire their 747-100 test bed as it was going to have to go through a major (expensive) maintenance check to keep flying (besides it was Pratt powered). However there was a short period when both the 747-100 and 747-400 test beds were available and GE found having flying test bed options to be very handy. I suspect they've decided that having a second 747-400 test bed available was worthwhile.