According to the regulations, "Space-Available passengers will not be removed in favor of other Space-Available passengers (same or better category)." So, this means, once you are manifested, (again, per the regulations) you should not lose your seat (at the originating or en-route station) to another Space-A passenger. That's not to say it won't happen so it behooves you to know the rules. However, you're not entirely safe once you are manifested. When necessary, Space-A passengers can be removed at the originating or en-route stations to accommodate Space-Required passenger/cargo. The order of selection for removal will begin with the lowest priority passenger with the latest date/time of sign-up as reflected on the manifest. If bumped and you choose to continue travel to your specified destination, you shall will compete again for seats with your original date/time of sign-up. For a full description of these rules, consult Air Mobility Command Instruction 24-101 Vol 14. Bottom line, regulations say you can't be "bumped" for another Space-A passenger but you can get bumped for duty/medevac pax or high priority (e.g. hazardous) cargo.
Once I'm manifested on a flight, can I be "bumped" and forced to give up my seat?