Since 1997 Bulgaria has operated under a currency board arrangement (CBA), which is an exchange rate regime where a country commits to keep its local currency to a fixed exchange rate against an anchor currency. For Bulgaria, that anchor currency is the Euro.
Given this, any discourse about how Bulgaria's entry into the Eurozone implies a dramatic change to its monetary sovereignty is likely unfounded. It didn't happen overnight - it's been an almost 30 year long process. Joining the Eurozone does, however, remove the pegging frictions for Bulgaria, and allows them to fully integrate into the monetary union.
Given this, any discourse about how Bulgaria's entry into the Eurozone implies a dramatic change to its monetary sovereignty is likely unfounded. It didn't happen overnight - it's been an almost 30 year long process. Joining the Eurozone does, however, remove the pegging frictions for Bulgaria, and allows them to fully integrate into the monetary union.