I want to say no, I'm almost positive the EX2 Plus ROM board has additional hardware on it. You can swap out the EX2 ROM board, but I don't think you can convert it
It's certainly worth burning some EPROMs and trying it though
Unfortunately I don't have neither board so I cannot physically try that
But if you are saying their ROM boards are different then they are not convertable, at least in simple terms, hence the price difference between them...
I can confirm that you can swap out rom boards on these ZN-2 boards as I bought a Rival Schools rom board and ran it on the bottom board that came from my Street Fighter EX2 Plus.
As far as EX2 and EX2 Plus goes, it would appear that all roms would need to be swapped out if its possible. According to what I can see the only chip that's the same on the top board is the coh-3002c.353, if that's the security chip then you are in luck but I just do not know enough about these boards to know. If this were ever possible I would mind converting my EX2 Plus to EX2 so I could play as Hayate as he was removed in the Plus version.
I can try a rom swap with legit roms when my friend actually sends me the EX2 Plus I'm buying from him. Protection chip is different so it might be a no go, but we'll see.