Hey @Ramsam,
It looks like you are working through a migration to fulfillment orders. We can share a few of our resources here, but there is also a developer community board specifically for questions around these changes Fulfillment API Deprecation.
Depending on your app making the change may or may not be as simple as swapping out one call/endpoint for another. Our first suggestion would be to look through the full guide covering migration to fulfillment orders. This covers process, outlines major changes in workflow, links useful docs, and helps differentiate between the main types of fulfillment related apps - Inventory Management Apps, Order Management Apps vs Fulfillment Service Apps. There are detailed guides for each app type - more info here.
I'd high recommend reviewing the docs shared above as they have many examples of how to manage, update, and create a fulfillments. However, the short answer is that our REST API the Fulfillment resource has been been update to follow the new process, but remains a way to create fulfillment on a fulfilment Order - as highlight in the order management app guide here.
Hope this helps clarify - cheers!
