For leptons (muons, electrons, etc.), the sum of the number of leptons plus the number of anti-leptons is a constant. So if a positron disappears, an electron will have to disappear also. So there is no "conversion". A negative muon can decay into an electron plus neutrinos. A positive (anti-) muon can decay into a positron plus neutrinos. When Fermilab creates say 10^12 anti-protons, there are 10^12 protons or neutrons also created. If you want to check on the number of anti-protons in the Tevatron right now, look at http://www-bd.fnal.gov/notifyservlet/www?project=outside