At some point you'l find the front 020 will cup badly. IIRC, mine started around 5K and by 6K I could feel it in the handlebars when cornering, and since I'm already half-way-'round-the-bend, I figured that would drive me all the way, so I changed both.
Others have run mixtures and report no undue problems. Assuming you are not a super aggressive rider, you should be okay, with one caveat: The Avons have a vastly different profile than the 'Stones. If you notice some "wonky" handling, change the front mui pronto!
If your riding style is more to the "touring" pace, you might not notice any difference.