If there is VAT/TAX rate invovled in your software

I am working with a project now which has TAX involvement. Here I got a scenario as follows.

1. VAT rate changed on 11 th of one month
2. The existing ERP cant handle item level VAT.
3. Multiple Invoice cant be made for one month

So this is my problem. We can solve it in two ways

1. Always keep the the VAT amount for each transaction
2. Or maintain multiple VAT rates field in your VAT table along with the date.

The first one is easy and safe.