I was able to get a one-way from BAH to CLT for 1/2 the miles on UA. I think the agent was mistaken to tell you that one ways aren't available international. However, the one-way deal for half-price is not available to use on their partners.

Plus, it's been my experience (I'm a 1P), that if I change the dates but not the routing or class for an award ticket, there is no charge.