Because of the precision fit involved, replacement wind cups come with an AirGap axle. But if you received your Wind Instrument after mid-January 2017, the replacement cups should fit your existing axle without needing to replace the axle. This is the second version of the AirGap Axle™ design, with further improvements on the original 2016 AirGap axle.
After putting the cups on the axle, give them a fast spin or blow, to make sure that the turn silently with no friction. If you do need to use the new axle, just pull out the old axle with pliers, add a drop of adhesive (e.g. Marine Goop, super glue or 5200) on the base of the new axle, and press-fit the new axle in.