You can change if there are none on sales order and none on purchase order and none on hand. Then you can change from Standard to FIFO. If you have quantity on hand, adjust out and then update, change to FIFO and then adjust back in. Use current date as FIFO date.

It's much easier to use the Sage IM - Utilities designed for this.  Data entry tables need to be empty (i.e. no unposted invoices / receipts...).

We have purchase order receiving and have sold the item with zero costs to a Job.

Is it a single item? Do you have any in stock?  If none in stock you should be able to go into Item Maintenance and just change it.

We still have item in stock with zero cost due to standard cost set up.

To correct the inventory value, you can use the Standard Cost Adjustment function before changing valuation (using the IM utility), or do inventory adjustments afterwards.