First check if it is allowed to change and what rules applies to do this.
This answer is just a suggestion, so first test in a separate environment before messing up!
It is possible change the inventory model on your item model group.
But you have to take care of correct postings. Except for Standard cost, you need te recalculate and close your inventory to have the appropriate costprice on the issued inventory transactions periodically. During daily postings an average cost is used for the inventory transactions. The real FIFO, LIFO or average cost will be applied by using the periodic closing and adjustment.
Try if you can close your inventory on a given date and directly after, change the inventory model to FIFO. Then create some transactions, recalculate the transactions and see if some scenarios are working correctly.
Again: first test, before you get in any trouble.