We have bins enabled for certain inventory items. They are already used on open SOs and POs. What is the negative impact of unchecking the Use Bins in the Item record while there are open SOs and POs for those items?

The only potential negative impact is if you already have inventory in the bins, them being on transactions does not matter.

It appears that it does not have an impact on existing and open SOs and POs. But when we will enable it again in the future and I am getting mixed results in my tests.

it stays in the bins when you turn off bins, and when you turn them back on they magically re-appear in the bins. the fulfillments/receipts matter for the bins, not the SO/PO

We plan to turn it on in the future. And I noticed that even if I wipe out the entire quantity through Fulfillments after turning it off, when I turn it back on hand, they are still in the bins although I do not have any quantity on hand.

yeah thats the risk with turning off/on while you have inventory

No impact to existing transactions other than the related inventory movement/GL impacting transactions. The inventory detail won't require a bin.

Thanks, everyone. What's the best practice here? Create an inventory adjustment to take them out of the bins before turning off?

If your goal is to turn it off for this item and keep it off, yes. Adjust out with bins on, turn bins off, and adjust back in.

I would test it - if I recall it may remove the inventory from the bins directly since it's not required or configured to use bins.