Under Bankruptcy Code Section 348, a conversion of the case, creates a new “order of relief” and the debts that are incurred during the previous case are now considered pre-petition debts.
For taxes, this does not matter so much since the definitions of “priority” are based on the petition filing date, which does not change upon conversion and the determination of discharge is based on that priority determination. So for the taxes you would be using the original filing date for your analysis.
Be aware however that other local debts (e.g. utility bills, parking tickets, etc.) are not entitled to priority and thus, those debts incurred after the initial filing would still be dischargeable using the Chapter 7 conversion date to determine that it was a pre-petition debt.