Frost cycles have the potential to move anything. However I dont think you will have much of a problem if you tamp the corner posts properly. That is to say you need to tamp them a few inches at a time starting from the bottom and work you way up tamping each layer before adding more dirt, pea rock or sand. Dont use frozen chunks of dirt, you will for sure have loose posts when the dirt thaws. If you can get steel T post through the frost, they will be fine.
One other thought if the frost is too deep you may need a frost bit to dig the post holes.