If you were going from Subscriber to Non you'd drop down to Preferred with 6 slots. 2 slots are only true F2P players. If you've still got a active subscription you can place a in game ticket to restore a character. I think character restoration is a subber perk. you can wait until you able to resub with again and try to get it restored then.
P.S.
When going NonSub all your characters go Inactive. Then you can pick and choose which 6 to use to play. Once you sub again you'll have access to the rest of them again until you unsub and get to pick 6 again. If not subbing again you can purchase more slots or delete an active character and you can reactivate one of the inactive characters. The only time you'd really need to delete characters is if you wanted to roll a new one. Then you'd have to have 0 inactive characters and an empty active character slot to create a new character
