If I was you, for a guaranteed business class flight from CDG to SKP, I would use LifeMile points to redeem it as an award flight.
That way you don’t have to purchase an economy flight on Austrian/Czech/Croatia/who-ever, on a possibly expensive fare that allows upgrades, and note that there is no guarantee your upgrade request will be successful.
Alternatively, Qantas and Virgin have bid-for-an-upgrade options. I’m not familiar with Austrian/Czech/etc. Do they allow bids? Note that when you total the bill of buying the economy seat and the upgrade bid, this may be more expensive than buying a business class seat in the first place.
In summary, your best option is to use your points to redeem for a business class award.