Seems to be a few posts about this for 2.x regarding changing order to tax exempt customer group. However, doesn't seem to be any solutions.
He is my problem. Customer came back and said we are a tax exempt charity. Can you refund tax? So I refunded tax amount. Setup Tax Exempt customer group and added this customer and my "test" customer to Tax Exempt group. Created order using test customer that I placed in Tax Exempt customer group. Everything seems to work.
Now here is problem... Edited order and changed order customer group to tax exempt. When you get to last step prior to saving, tax is still added and after saving order, customer is still on default customer group for that order. Any ideas on or solutions?