Cliff, due to the fact that nothing significant has changed in the motor like, Pin size, squish chamber, valve lift, duration and angle, they should be good to use.
Pistons are better used in pairs but if there is not more than the odd gram in weight difference and you have one of each new, you would be able to mix them.
Even if the bore stays the same, don't mix them if one has been used and the other one is new.
Sam.
