A question though, since I've never changed a cable slide, is this something you need to do with a press or can you change it without one? Also, will it change my current tuning process?
As said, you don't need a bow press with the Guardian. The part of your question that was unanswered, will it affect your current tune, the answer is 'probably.' Switching cable slides doesn't always change a bow's tune but it often does.