It's all over the photo kit news today. Sigma announced they will soon start offering a service where they convert the lens mount for a fee. Only applies to their latest lenses and mirrorless ones.
While it is nice to have the option, I wonder how much use it'll get. For low value lenses you may just be better off replacing it. But it gives more options on higher value lenses if you change system. Are you that likely to change DSLR system though? While it addresses Sigma lenses, you may still have a load of other branded lenses.
About the only other possible benefit I can see is if you want to dabble with Sigma's own bodies and their Fovenon sensor. Get Sigma lenses for that, and if you later decide to end using the system you can convert the lenses into something more common without having to find another Sigma user to try and sell them onto!
_________________Canon DSLRs: 7D, 5D2, 1D, 600D, 450D full spectrum, 300D IR mod
Lenses: EF 35/2, 50/1.8, 85/1.8, 135/2+SF, 28-80 V, 70-300L, 100-400L, TS-E 24/3.5L, MP-E 65, EF-S 15-85 IS
3rd party: Zeiss 2/50 makro, Samyang 8mm fisheye, Sigma 150 macro, 120-300 f/2.8 OS, Celestron 1325/13
Tinies: Sony HX9V.