Can I decrease a photon's wavelength by a medium or a vacuum? Are there other ways of decreasing the wavelength?
Just to mention the basics for "other ways"', you can hit your photon really hard with a fast moving electron or proton, i.e. inverse Compton scattering. ICS is very important in many astrophysical contexts. If you even reflect the photon off a moving mirror, you can slightly increase or decrease its wavelength