I came with an easy, cheap solution. Instead of moving the thumbtacks, you just move the posters.
You'll need just a couple of supplies and you can be very cheap to very creative on this or anywhere in between. I wanted to try it out and make sure it worked before I got fancy.
flat-topped thumbtacks (these probably have a name)
I used whatever my husband had in this contraption here but I don't want you to think that you have to run out and find this. Use any glue that will hold, I suggest a glue gun or plain old white glue
Total cost for one poster holder:less than $1
If you're going to paint, let the glue dry and paint now, before adding the tacks.
Stand the contraption upside-down to dry.
Now just decide where you want to hang your posters, use a level to make sure that it's straight. This makes changing out posters and artwork a snap!