Is that now the villager hits your guild guard, you spell up, he runs away for 8 hours, them comes running back, knowing you are now without your best protections.

If you leave the barrier source still in in wand form, then this is not as likely to happen. I would make Barrier completely incompatible with the built-in spell. Maybe make Aura/Shield work like they do today with barrier, but like you suggested, have aura/shield less effective with the built-in spell. This would also be a incentive to explore to find your barrier source.

So, I guess what I am saying is I like the build up spell idea, but to encourage exploration I would keep ABS as well.