Graham,
Fully accepting your preference for using a knife, I can't help but wince slightly at the thought of using one for this purpose. My first concern would be for the risk of drawing blood from one's fingers in the event of a slip - with the consequent risk of bloodying the watch and strap. Having acquired a good spring-bar tool (at very modest cost) I find that it makes the task of changing a strap so much more relaxed and enjoyable; and feeling fully under control, it makes the chances of slipping and damaging either oneself or the watch so much less.
Also, as a small-time knife collector, I'm also thinking of the potential for damage to the blade of a perfectly nice knife!
;-)
Michael