I decided to call WP and ask them, if I were to move back to the US at some point, what the conversion in the reverse direction would be.

They told me that this was a one-way offer only and if I ever moved back to the US I would be unable to transfer my FD points to WP. I would need to keep a European mailing address and use my points through their program.

I take that with a grain of salt, but I still decided to stick with WP.