The tax on the sale of a carbon emission allowance is not changed by this measure. The measure only changes who is responsible for accounting for the tax. It used to be the vendor who collected the tax from the purchaser to remit to the CRA. Now it will be the purchaser who will be responsible for self-assessing the tax and remitting it to the CRA if there's a need for that.
I want to add here, to clarify, that the purchasers of those carbon emission allowances are usually businesses involved in commercial activity, and they would be entitled to recover the tax through the input tax credit mechanism.