For many clubs, relegation brings about a whole catalogue of changes; especially when dropping from the Premier League into the Championship. The difference in income between the two tiers of English football is unbelievably vast, and as much euphoria as it brings clubs heading into the top flight, that change can be equally as devastating to teams on their way down.
And that's exactly why you get wholesale changes - playing staff leaving, budget cuts wherever possible. There's no more of that sweet gold mine in TV rights deals the Premier League has, and changes have to be made accordingly.