Most speaker mods involve things like crossover component quality upgrades, internal wire upgrades, or binding post upgrades, in addition to various cabinet-deadening strategies such as coatings, adhesives/sealants, or sheet appliques. Some folks go as far as changing, removing, or augmenting the cabinet stuffing materials, but both that, and any driver changes done in isolation (in other words, not as a manufacturer-sourced upgrade in conjunction with adequate testing and concurrent crossover mods as needed), will stand a very good likelihood of changing the sound in unintended - and probably detrimental to accuracy - ways. The only other things commonly done involve various stands or footers. I agree with Phasecorrect: If you want an upgrade here, you're better off looking for different speakers in most cases.
Is it taboo to mod a speaker?
I often read and hear about modifications done to source equipment, amplifiers, DACS, and ocassionally I read about someone doing stuff to the crossover portion of a loudspeaker. I myself plan to have someone modify my Pioneer DVD in a few months. However, I rarely read about anyone modding a loudspeaker by exchanging drivers. I have never heard of anyone exchange or upgrading to driver ABC from drive XYZ. Is it done? Does anyone do this professionally?