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Four ways:
1. Relocate the primary mirror up the tube to shift the focus. A no-brainer because it means no change to the focuser and diagonal, and all it takes is a drill and 3 holes.
2. Revise/replace the focuser. $$$ but could be worthwhile if you want to add the entire image train (filter wheel, rotator, EAF).
3. Use a "sled", in which the camera and secondary mirror move along the OTA together on some sort of sliding arrangement. Has been done, there are examples of this. The advantage is there is no traditional focuser, so vignetting is reduced, or you can use a smaller secondary mirror.
4. Rebuild the whole OTA so that you have a fixed focal plane (no focuser at all) and focusing is accomplished by moving the primary. A bit of an engineering challenge, but it's doable, and has been done.
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