I have the Sigma f2.8 focal length 24mm Super Wide 11 lens which I used on my Canon FD cameras and found to be very good. Question: Can I change the breech lock mount for a Pentax 42mm screw in order to be able to use this lens on my EOS20 via a Pentax adapter?
No, you can't do that. Unless there's an adapter for it you're going to have to buy specific lenses for each platform.
While in theory you might be able to disassemble everything, replace the electronics and mechanical components specific to one platform with those for another, that's a major job and you're extremely unlikely to succeed at it unless maybe you're an experienced optical technician and electronics engineer. Don't try at home...
You're basically asking if you can use the board computer of your 2014 Honda Insight in a 1980s BMW 3 series by changing out the connectors...
Sigma is now offering services to change the lens mount, you can read more about it on the "Change Your Mount" page. The only problem is that they offer it for a limited number of lenses and 24/2.8 isn't in that list.
Your best bet would be to get FD - EF adapter with optical element inside, so you would be able to retain infinity focus, although none of the lens electronic components will work, so you'll loose autofocus, aperture adjustment, etc.