If you have an e-ticket saying "Business" but you end up in economy, then probably you have a case, however not one under EC261 (where there is a clear reimbursement protocol) since this flight sector is out of scope for EC261. It should be pointed out that under EC261, a refund is a remedy and this has been offered here. The other remedy under EC261, a rebook, isn't on the table. So I am sceptical you can do much about this other than cancel, retrieve the miles and buy a cash ticket instead. The exception would be if the Polish small claims court can be interested in the case. In the UK context you can try the argument "ok it's not EC261 but the EC261 process indicates that course of action X is the consumer friendly protection here". But this is quite tentative really.