The Regal F-ED series spotters have a 1.25 in eyepiece holder and the OEM-supplied zoom eyepiece is easily removed from the scope by a twist of a knurled ring. Because 1.25 in barrel-diameter eyepieces are very widely available, you can use other eyepieces with your Regal.
However, not all eyepieces will work with the scope because of the design of the internal focusing and 45-degree prism assemblies. This places the image plane for the spotter’s objective at about the back of the knurled ring. So the Regals have a small back focus distance.
Eyepieces that have long 1.25 in nosepieces or barrels (with field lenses recessed about 1 in or more) generally won’t be able to achieve infinity focus with the Regal spotters. Nor will 1.25 in type T-adapters or Barlow lenses work with the scopes.
The OEM zoom eyepiece has both a short nosepiece and its field lens at all magnifications is close to the end of the nosepiece tube. Eyepieces that will work with the Regal will have similar physical characteristics. As an example, most eyepieces of the Celestron X-Cel series have these characteristics and will work with the Regal F-ED series.