I've always figured that the basis of a Werewolfs longevity would come from the fact that, if a shift restructures the body, it would also heal whatever damage has occurred.
If you still want a quick change, a cascading virus would do it. The idea is that a few viruses invade a few cells and cause the cells to excrete triggers that allow more viruses to invade more cells in geometric progression until all cells are infected.  ¶ 9:12 PM