Should be no problem whatsoever. Just save the old speaker so you can put it back in the event of a warranty claim. It might technically void the warranty, but I bet nobody will ever notice or care.
I guess my only concern would be: Speakers "break in" and take about 10? hours of loud playing to reach the tone that they were designed for. With only 1W input to a speaker designed for 10 or 15W, the break-in process might happen slowly or not at all. However I guess Weber's "pre-aged cone" could be considered pre-broken in to some extent.