In that case you should check out Can’t set nameservers - (Inconsistent Set of NS / Nameserver not authoritative) however option #1 won’t work for you. You can only follow through on either #2 or #3.
Of course you can change the nameservers back to your host’s too and simply wait until Cloudflare drops the domain from the previous account, but it will most likely drop it from the new account as well. So you’ll have to re-add it at that point.