We are laying some oak tongue and groove flooring amidst the current old oak boards. I need a precision tool that will cut ONE board in the middle of other boards. We do not want to destroy the surrounding boards. A circular saw will cut more than one board. I am considering a plunge router but I do not know what bit to use to get a straight cut.
I've always used a circular saw for this. Accuracy isn't an issue. You're going to make 2 parallel rip cuts in the board about an inch in from each side to stay away from the nails. Get to within an inch of either end. Take a sharp chisel and complete the 4 end cuts. All you're doing is splitting the wood to the end. Once the 4 ends are split, lift out the middle piece. The groove piece will pop out as it's not nailed in and you'll chisel out the tongue side from the cut to the next board, careful to avoid the 1 or 2 nails.
Cut the new board to length and remove the lower groove piece and drop it inplace. You can face nail it or hold it down with adhesive.