Parashath Noah, Laws of Ziziyoth, Page 15-16:
8 (continued). If the tallith becomes torn across more than half its width or breadth, the Poseqim HaAharonim rule that one must first untie the ziziyoth, and only affix them again, once the tallith has been repaired (cf. parashath Lech Lecha, 10).
(The reason for this is because, if the tallith becomes torn across more than half its width or breadth, it now clearly has more than four corners, and we must take into account the opinion (above, 6) that it is now exempt from ziziyoth. If the ziziyoth were still attached whilst the repairs are made, they would be pasul as this would involve preconstruction.
Although the final ruling does not follow the opinion that a tallith may not have more than four corners (above, 6) and Maran z'l does not accept it either, the Poseqim HaAharonim concur that it should be taken into consideration whenever possible.
A situation where this problem could easily arise is when a tallith is made up of two pieces of material, and the seam may come unsewn, or tear, until the tallith is torn more than halfway across.)
This learning should be in memory of Maran, HaRav HaGaon Ovadia Yosef, ztz'l.