I would need to see the nocks, and the new string. If its plain old self nocks you would really be risking damage. If it has overlays its probably ff compatible(if properly made).
You can make a fastflight string with more strands, or pad out the loops to reduce the chance of the string cutting into self nocks. By doing this you reduce the weight/speed advantage that FF has, over older material strings.
If in doubt use Dacron