A big change for you for sure! To ease the pain, consider training courses from your partner or a training company that specializes in business central. They cost but they will really help.
So changing the cost--I'm assuming that you have transactions for the item that needs the cost changed. BC has a Revaluation Journal that you can you to change inventory cost. You will want to change the cost on the transactions that add quantity into inventory. Don't worry about the deductions from inventory, run adjust cost after you finish your revaluation journal and the system will from the changed cost through from positive to negative.
You will want to be aware of the date(s) that you are using in your revaluation journal. and I suggest you setup a test company and run a trial before doing in your 'live' company.
