I would amend your return to include a simple statement saying pretty much what you shared:
"Inadvertently chose Option B on Line 33 in Schedule C 'lower of Cost or market value' when I have always used Option A 'cost'. Amending return to reflect the Cost method." Include a copy of your corrected Schedule C with the 1040X.
It is smart to be proactive about this as you need to use the same inventory valuation method from year to year. You can change your method of valuing inventory, but then you need to file Form 3115, change in method of accounting. The IRS may send a letter about your inventory accounting method change if you don't send an updated Schedule C.
