change Vat

i want change the vat percentage from 10% to 13% but there is a lot of sales is not posted

and this un-posted sales with vat 10% .

the question if i change the vat before finish all pending sales , the system will take 10% or 13% vat for the un-posted when posting the old transaction

or i must finish any un-posted transactions before changing the vat ?

As your question is not clear, assuming that Vat rate changed on a certain date.

In the Vat - Sales Tax code create a Value of 13% by giving the periods and restrict the value 10% to date.

So when you are Invoicing it checks the invoice transaction date and if it falls in the range of 10% then it will pick 10% and if it falls in 13% then 13% will be calculated.

Hench you need to take care the Invoice Date to Calculate Old and New VAT Values.