Same thought here. New Hampshire used to be reliably conservative, but liberals moving there from Vermont and Mass. (to escape high taxes, what irony) have moved it to at least a toss up, if not fairly blue state.
The conclusions reached in this article could be true if liberal blue staters move to red states and are interspersed among many heavily conservative districts. In that case, the population shift would lead to more congressional seats for the red state without necessarily impacting the heavily conservative districts.
Also, it's a probably the case that many people with the means pick up and move to a southern (red) state would be those wanting to escape the high taxes of blue states -- which means they might lean conservative to begin with. Otherwise, they'd be delighted to stay in blue states and pay out the nose for all their beloved government programs.
The Northeasterners moving south seem tobe leaving their ideology behind them. The Californians trying to escape the insanity that is California are carrying the ideology in their UHauls right along with the furniture. It has given rise in Idaho and Colorado to a new concept-californication.