Backward Constant Interpolation for Keyframes

I’m animating an armature. For various reasons, I’ve done animation work for later points in the timeline, and am now adding animations earlier in the timeline. However, the problem is that when I advance past the last keyframe in the earlier animation, the bones start shifting toward their positions in the first keyframe of the later animations.
Is there something I can do to prevent this from happening (so that the starting position for bones I’m animating in-between the already-animated segments is always the position defined by the prior keyframe)? I’d rather not have to repeatedly set and then remove constant interpolation from whatever is the latest in the set of earlier keyframes.

Use another action. You can copy the keyframes to different actions and delete the ones you don’t need. I usually push some down to the NLA and then mute those I don’t need. If I want to see what I’ve done with the others I just Use the NLA, but you can always copy and paste keyframes between actions if you need to.