I have tzitzit strings on one corner only that have torn. Can I simply replace that corner and tie on new strings, or must I undo all the strings? In other words, is there a taaseh v’lo min ha’asui in this situation?
If I understand your question correctly, the strings of one of the corners of your tzitzis have ripped, and you want to untie that corner and replace the strings and retie them. This is not taaseh v’lo min ha’asui because this the process of making tzitzis. Taaseh v’lo min ha’asui is a problem when a mitzva is completed without making it, rather by causing that it should be automatically done, such as by digging out the inside of a pile of wheat and automatically there is a roof, even though it wasn’t placed there. Or when placing an already finished corner of tzitzis and sewing the completed corner on to the garment. However here you are actually making the and tying the corner. Besides, even if you were to remove all four corners, after putting on three you would be left with one corner to finish. Therefore it is not a problem at all.