Anything you've already paid for is yours to keep. You will get the sub bonuses for the full 6 months and retain all the crowns you were given (until you spend them). This also means you will not get any money back for cancelling it.

When that 6 months comes to an end the bonuses will be removed from your account, any character in a DLC map which you haven't bought from the crown store will be kicked to a regular map, materials will no longer go into your crafting bag, you will be unable to dye your costumes and the item limit on your houses will be halved.

But any costumes you dyed will keep their colours (you just can't change them), any materials in your crafting bag will stay there and can be removed whenever you want (you just can't put them back) and any items already in your houses will stay.

Whereas if you didn't cancel it another payment would go out of your account for another 6 months, another load of crowns would arrive and all the bonuses would continue.