This will likely come with a one-time significant increase in inflation, at least based on other European countries. When Germany converted to the Euro, the conversion rate was (IIRC) about ~2 DM to the Euro but from what I recall, a lot of everyday things went from costing 7 DM to 7 euro, effectively doubling in price. IIRC France was similar (ie ~6.5 francs to the Euro but 10 Francs went to 3 euro, etc). I've tried…
The country already experienced quite a bit of inflation last years, regardless of not being in the euro. I don’t see why the change of the currency that is already pegged to euro since the creation of Euro will cause any inflation beyond the rounding and the rounding is for 1:1.95583 and that often provides rounding sown incentive as 4.99 becoming 2.04 and 4.49 becoming 2.2957
A more realistic concern can be that Bulgaria might start borrowing irresponsibly. Currently Bulgaria's debt to GDP is just around %22, which is very low.