Not able to assign input tax code in GL account after posting balences

Hi Experts,

I created a tax GL account I posted the balances to this account, now I want to make settings in control data Tax Category field as a input tax account and I want to select post automatically, now system is not allowing to make this changes.

If i make this settings first I am not able to post this account, after posting balances system is not allowing me to do the required settings.

Please advise me how to proceed.

Thanks

Srini