Can I swap soy milk for cream in a chicken pot pie?
Yes, you can use soy milk, but it will definitely affect the flavor. If you like the flavor of soy milk, go ahead, but I don't find it that appealing. 

Instead, I suggest using full fat coconut milk or coconut cream (you can find it in the international foods aisle in most grocery stores). I have used this subsitution several times before, and it will give a hint of coconut taste, but I think the overall flavor for your recipe would be richer.

Another alternative is to just use an additional 1/4 c. of broth and leave out the cream or any dairy substitute all together.

Edit: I'm not 100% sure on the texture, but soy milk is fairly "watery" compared to cream, so, again, I think a richer/fattier "milk" would be better.