Changing tax will affect older GRN

Hello,

I want to ask about changing item or supplier tax. It seems that if I create a PO or GRN and then the tax setup is changed in Item Tax Types, the tax in already created PO or direct GRN will also be affected.

For example, if I create PO yesterday which only consist of item A and item A only has 1 tax,  it will calculate the overall amount using that tax. But, if I add new tax for A today, when I check the yesterday's PO, the overall amount is also changed which now has 2 tax for item A.

It is make sense from the coding perspective because PO only refer item id and it will query tax information from the database according to what is stored when it is access. But isn't it dangerous? Old PO or direct GRN should retain its tax information when it is created. So the invoice should be based on the tax information when PO is created not when invoice is created.