Look into the requirements of standard cost (Cost Plans, revaluing inventory etc.). There is a fair amount of maintenance that exists when using standard cost compared to Average. Anyone I have seen make the switch is due to the effort required by the finance team, and just having a couple people who understand the full process is a challenge vs. the value you get on reporting with standard cost.
If your items are already configured with a different costing method you will need to recreate with all of the item records, sounds worse than it actually is - Export full item list, inactivate and rename old items, reimport with correct costing method