You cannot Change, because that would result in Data recalculation and Chaos.

You Transition.

Edit the Service items to put a Z in the name, first character, if you will never use them again. Here is the assumption that these are Two Sided Service items, always listed on the Purchase, so everything flows to COGS from purchases.

Make the inventory item names you want. Set the price. Do Not put in Quantity on hand. Get all set up.

Take a physical inventory. Quantity and Cost on hand by Product being managed as Inventory.

Make one large Adjust Inventory screen, selecting all Products on hand, using the physical inventory info to Quantity and Cost what is on hand. Top left, pull from COGS; it is Too High, because these are still On Hand.

Now stop using the Service items. Make one Parent Level and name it Old Sales Items, slide them all under it, and make all of them Inactive. Now they stop showing in dropdowns for selection.

Now you are transitioned.

That's a general overview.