I just looked this up for my own harvesting.
The IRS has some ambiguous language. You can't buy the sma fund but you also shouldn't buy a similar fund with a different name. Ie, if you swap a vanguard s and p fund for a fidelity s and p. You can swap a total market for an s and p. That's what I did. You also can't rebuy the fund you sold. So if you have auto buys on your 401k you need to switch them.