Celtic FC and Cardiff City have reached an agreement that for the 2017-18 and onwards they will be swapping league associations. Cardiff City will now ply their trade in the Scottish Preamier League, and attempt to qualify for the champions league through that manner. Celtic however, will now play in the Sky Bet Championship, with the aim to be that they will become a powerhouse in the English game.
Celtic shall retain their place for this season's champions league season, but afterwards they will compete as part of England's representatives.
(AN: I will only manage Celtic, but will add details of Cardiff's journey as well here and there.)