Problems with this plan:
1. Harris is a good tackler for a corner, he'd be average for a safety and isn't big enough to consistently hold up between the tackles. What do you think a quality FB would do to the kid?
2. Nickel corner is a pretty high value tackling position as well, FYI, which is why everyone noticed Harris' tackling skills in the first place. A lot of times the nickel corner has first crack at the RB on running plays out of spread looks.
3. The nickel corner blitz has been one of the few very successful blitz packages we've been able to run. A big part of that probably comes from Harris being in that role. We shouldn't be dicking with one of the few ways we've found to hit opposing QBs.
4. Harris has the speed and quickness to hang with quality slot WRs. That is already incredibly valuable in the slot, and bodes well for his eventual transition to the wing as a full time starting CB. The value of those tools is only reduced by moving him to safety.
5. Harris jams better than every CB on our team not named Champ. Pretty damn important tool for a nickel CB to have, not nearly as vital for a safety.
All in all, it's putting a square peg into a round hole just because it fits that round hole better than all your undersized round pegs. The solution isn't jamming him into a worse spot for him, it's getting more high quality square pegs.