Replacing bike frame on BMC Alpenchallenge AC01 I have a BMC Alpenchallenge AC01 IGH Alfine 8 Speed 2017 size small. Although the size is appropriate for me, I would prefer the medium, but the small was 47% off, so I bought it.

Now I have the chance of buying the medium size frame of the high end model. The frame is from the same year, but it came originally with Alfine 11 and comes with a carbon fork.

Do you guys think I can simply replace the frames without much hassle? I would guess I would only have to take the components from the small and mount them in the medium frame. I would really appreciate some opinions on this. Thank you very much.
If the new frame is the same frame model and year (the aluminum Alpenchallenge frame, that was used on a higher spec bike, and came with a carbon fiber fork), there should be no compatibility problems. Seat tube diameter, steerer diameter, bottom bracket shell, disc mounts, hub widths etc. should all be the same.

I note that the Alpenchallenge comes in both derailleur and internally geared hub versions. The internal hub version has a means to spit the drive side seat stay to install a Gates Drive belt and a eccentric bottom bracket shell. Obviously, make sure you are getting this version.

Other issues you might run into:

If the chain stays on both frames are not the same length you might have to get a different length belt.

You'll probably have to replace rear brake and shifter cables and housings as the run length will be slightly longer.

The medium size may have come with a slightly longer stem than the small, but that may not be a problem for you.

Don't forget to check for any special tools you might need.  Bottom brackets typically need installation tools, but I'm not sure about eccentric types.