simple,as long as your new shower tray sits at the same height to the underside of the tray and you use the same or similar trap,all you have to do is measure waste hole centres on new tray,mark floor and repipe waste to suit,use mcalpine fittings to alter pipe as these fittings 'give' a little and will allow for slight misalignment of trap.It can be difficult getting trap lined up perfectly.remember and silicone trap surface and rubber washer before setting tray down,(not always necessary but easier than lifting out tray if there's a leak).
steeveepee, November 2008