Most mystery is written in a single third-person voice using a single POV, although in recent years there are more exceptions than in the past. So if you do what your beta suggested, you increase the likelihood of pleasing a publisher.
I cannot stress strongly enough that you begin by making a copy of your mss. and changing the copy, leaving the original first-person just as it stands. Why? Because you may decide that it was better before you started messing with it.
When I've changed voice from first to third, it takes many passes. The first one is the longest, or seems to be, because it's literal drudgery: One sentence at a time, change first person to third. I becomes he or perhaps she. Me turns into him or her. While a global Find-and-Replace would be great, it will change things you don't want changed (I used in dialogue, him referring to someone other than the POV character, etc.) You may find that after a half hour of this, your brain stops working and you have to take a break.
While you're doing that step, you will probably see other things that need changing, not necessarily because of the voice thing. Write yourself comments in the document, or notes.
The next step will be addressing commentary your beta reader gave you. Since there are plot changes recommended, this involves rewriting parts--in third person.
Next pass, you clarify the characters' as unique, making them behave and sound different from one another. Remember, every character is the star of the show in their own POV. You can make even small characters more rounded if you try.
After that, I'd put it away for a few months, long enough that when you take it out again you can read it as if someone else wrote it. You'll see all kinds of things that could be improved.
Maryn, knowing this is a long slow slog because she's done it
