It's actually quite common in potential forms to seen an を replaced with a が. I'm not sure if there's a rule for when, but when I was learning it our teachers pretty much made us stick to using が with potential, and definitely always use が with できる. For example: テニスが出来る - （テニスを出来る would be wrong even though the non-potential form would be テニスをする ). Some Googling has led me to believe that を with potential form is not standard, but is becoming a little more widely used.