Hey @Ramsam - if you're creating a fulfillment you can use this API Call in REST or this one in GraphQL. There are a few more answers about how the new Fulfillment Orders framework works here in this post. Briefly though, location IDs are tied to specific fulfillment orders now, rather than the fulfillment object itself.
That said, one way you could change the location_id for a fulfillment order would be to use the fulfillmentOrderMove mutation in GraphQL (REST equivalent here). This would change the fulfillment order's location to the one you want to use. This would also depend on inventory availability at the new location.
Hope this helps - let us know if we can clarify anything on our end.