Bread recipe calls for dry milk powder. Can I use dry buttermilk powder? I'm making a white pain de mie. I'd like to try to use up some buttermilk powder in place of the dry milk in the recipe.

I've swapped out some of the water for fresh milk to substitute for dry milk before without any ill effects. 

The dry milk has a bit more protein than the dry buttermilk, and the dry buttermilk has a trace of fat. I'm not finding any other relevant differences in the nutritional facts. The reconstitution ratio is the same. 

The ingredients of the dry buttermilk are: A cultured blend of sweet cream churned buttermilk, sweet dairy whey, and lactic acid.

Should I expect any problems just swapping them out, 1:1?
You'll be fine doing a straight substitution.  Well, maybe I shouldn't be so definitive since these are powders we're talking about and it's not quite the same thing, but I've substituted buttermilk for whole milk in bread recipes plenty of times.  The only thing that changed for me was the rising time.  When I use buttermilk in my sandwich bread, it needs an extra 20 minutes or so on the first rise compared to the whole milk version.