I managed to do it by using the MemberSavingNotification and using the member service to get the old member details to compare against:

public class MemberSavingNotificationHandler : INotificationHandler<MemberSavingNotification>
{
    private readonly Lazy<IEmailService> _emailService;
    private readonly IMemberService _memberService;

    public MemberSavingNotificationHandler(Lazy<IEmailService> emailService, IMemberService memberService)
    {
        _emailService = emailService;
        _memberService = memberService;
    }

    public void Handle(MemberSavingNotification notification)
    {
        foreach (var member in notification.SavedEntities)
        {
            if (member.IsApproved)
            {
                var oldMember = _memberService.GetById(member.Id);

                if (oldMember != null && !oldMember.IsApproved)
                {
                    // send an email if user goes from not being approved, to being approved
                    _emailService.Value.Send(
                        member.Email,
                        "Email Subject  - Account Approved",
                        "Email Body");
                }
            }
        }
    }
}