I have many devices that have been inserted with skipFcntCheck to true, what would I encounter if I now wanted to make a massive change to set them to false? Do I have to do any actions on the devices later? or is it enough to reinforce a join even with a downlink?
Assuming your device firmware is operating correctly (sometimes a big if, it will depend on your device), there shouldn’t be any noticeable changes. Flipping even a large number should be pretty quick with API calls.
That being said, I prefer to approach this by not turning on the skip in the first place, other than short-term debugging of an encountered problem.
Keep in mind that after changing this setting, the device needs to re-join before the change comes into effect. This is because this option is used when looking up the device-session given a PHYPayload, and this step comes before the rest of the data is loaded from the database.