AWESOME! Congrats on your successful session!
At this point, yes, you will lose your back up. The reason for that happening is because right now you are simply asking him to reach for the bit when he feels contact. So after he is very, very good at reaching for the bit from the halt, walk, and trot, then you can ask for the back up again. This was something I had to be very patient with, considering my horse's confidence issues. Basically you can start from the ground while you have the bridle on him IF you use voice commands. If not, re-teach it from the saddle.
This is how I distinguished my back up with my horse. You ALWAYS want to start with your seat and energy first, not your reins. So bring your energy back, get a backing up kind of feel in your body, then pick up your reins and hold one steady while you RAISE YOUR OTHER STRAIGHT UP. Not back, but lift straight up. This is called a Suspension Rein. What this will do is give the horse the idea to elevate his shoulders and come back. Now, you don't have to lift the rein waaaaay up high, that's not the idea, but lift it up some. Now your horse may try to reach for the bit so when he does go with him to some degree but then just hold. It may help to face him toward a fence so he really can't do anything but back up. When he even shifts his weight at this point release and rub him. Reward the slightest try.
It's really cool how horses can distinguish between things like this. When I started re-teaching my back up Rango actually caught on rather quickly. And it was wonderful to feel him be soft and confident in the bridle when I backed him. His cadence was right on, soft in the poll, so much different then he was before.