If you run summer tires in Croatia then if you may get fined by the police if your vehicle is not legal to use. If you have an accident, and it is snowing or icey at the time, the insurer could argue that it was as a result of the summer tyres being fitted. They could then try to repudiate the claim, but they would be on very uncertain territory. They would have to prove it was as a direct cause of the tyres. I personally would leave them on, most modern winter tyres will be perfectly fine in the higher temperatures, though may wear quicker due to the softer silica content. If you run them a few PSI higher than you normally do, they should be fine.