It sounds as if your bolt hold open isn't being affected by your magazine. Some metal magazines didn't have followers that would actuate it. Your simplest solution would be to just get a different magazine...anything currently available should work.
This is assuming your bolt hold open is functioning properly....if it never was cleaned out of it's cosmoline, years of shooting with a mag that never touched it...could have it a little sticky....it's possible that might be the entire problem. The magazine exerts it's least amount of pressure when empty, if the follower doesn't have enough oomph to push up on the BHO....it would close over the feed lips and make mag changes difficult. If it were functioning properly, the bolt would stay open on an empty mag, and you would be free to drop the mag.
I would suggest leaving your Romanian alone, skip the bolt modification... you likely would regret it..or pass it on to someone who would regret you doing it.
You could look around for a new bolt, have it headspaced and do the mod to that....sparing the original from permanent harm.
You can train yourself to leave the gun unmodified, and do mag changes (with the BHO working for you) fairly easily...sure, it's not the smoothest...but it wasn't designed with removable mags in mind, so it's going to be a little wonky regardless. You can manually actuate the BHO with your finger, insert a mag, and while pushing down on the cartridges, pull back on the bolt, let the BHO drop, and carefully slide the bolt over the rounds as you slide your thumb out of the way without chambering a round.
Your truly best bet would be to go back to the fixed ten rounder...and getting proficient with stripper clips. If you need more than ten rounds for your nocturnal bumpage....you likely need a bigger gun anyway.