If the chickens are on the land right now, and you want to put turkeys on in the near future, then they are at fairly high risk of blackhead. Do you worm your chickens regularly? I personally would leave land a minimum of 12 months chicken free before moving turkeys on, allowing the grass to regenerate and then mowing it regularly which allows any worm eggs to be exposed to the sun which will destroy them.
I'm assuming you are talking about poults/growing turkeys? Mature adult turkeys are at less risk as their immune systems can cope rather better.