Hasher that will fall back to legacy hashing.

FallbackPasswordHasher.cs

using Microsoft.AspNetCore.Identity;

namespace YourProject.Infrastructure.Security
{
    public class FallbackPasswordHasher<TUser> : PasswordHasher<TUser> 
        where TUser : class
    {
        public override PasswordVerificationResult VerifyHashedPassword(TUser user, string hashedPassword, string providedPassword)
        {
            var result = base.VerifyHashedPassword(user, hashedPassword, providedPassword);

            if (result == PasswordVerificationResult.Success)
                return result;

            if (LegacyAspNetIdentityCrypto.VerifyHashedPassword(hashedPassword, providedPassword))
                // NOTE: You can either accept the hash as is, like
                // return PasswordVerificationResult.Success;
                // or return that a rehash is needed (to update the hash),
                // this will look like the first attempt is a fail, the second attempt will be a success
                return PasswordVerificationResult.SuccessRehashNeeded;

            return result;
        }
    }
}