Can I edit a video that was shot in 1080p in after effects in 720p, but then bump it up to 1080 before i render?
You may want to try asking this in the After Effects forum.
In general though it is best to do your editing at the final output size desired, and only temporarily degrade quality using the resolution control for better interactive performance.
If you're asking about Character Animator, maybe describe in more detail what you're trying to do between Ch and AE and I'll try to point you in the right direction.
Yeah, you CAN, but if the highest-possible image quality is the priority, it isn't recommended. I'd precomp the 720 edit, put it in a 1080 comp and apply the Detail-Preserving Upscale effect. It ought to look pretty darn good indeed... just not absolutely perfect.