Change a Sigma lens mount myself?

Is it possible to buy a Sigma lens with a canon or nikon mount, and then switch out to a sony/minolta mount?

I was just wondering if anyone has tried this - without using the rather expensive Sigma conversion service.

Not really.

The only source for the mounts would be Sigma and I doubt that they would sell you the mount (even if there was such a thing) unless you are an authorized Sigma repair shop

Just contact Sigma and ask them.

The problem you may not be aware of is that the lens mount flange to focal plane of those three cameras are all different.

NO!
Lenses are very high precision items and it would be impossible to do the job without the correct tools and environment.
Why not just buy a Sigma lens with the mount you want.

Not possible.
It is akin to buying a car with an automatic gearbox and subsequently trying to convert to manual/stickshift. You can have either when you specify from new but attempting to convert one to the other is a massive task best achieved by disposing of one and buying the other.

You need more than the mount - all the electronics will need reprogramming and rewiring to talk to the electronics in the camera.

Nope.
Tamron used to sell a set of lenses that had what they called the "Adaptall" mount. Those lenses were specifically designed to have the rear mount section be user-removable and changeable -- you could buy several rear mounts for several different camera brands, and swap them whenever you felt like it. I have a 28mm f/2 Tamron Adaptall lens, and I have mount modules for Pentax screw mount, Olympus bayonet mount, and Nikon bayonet mount.

The Adaptall mount went away when camera manufacturers started putting electronics IN their lenses, and electrical contacts on the lens mounts. It simply was no longer possible to make the electronics IN the lens work with all of the different signals and protocols that manufacturers were using to talk to their lenses, so third-party lens makers like Sigma and Tamron had to start making versions of their lenses specific to each brand/mount, with different electronics in each of them.

Sigma would not only have to swap out the mount on the rear of the lens, they'd also have to "re-chip" the lens with a new electronics module that would make it work on the different kind of camera. You can't do that yourself.