My Doblo, an '03, came with 14" wheels as standard. 15" became the new standard with the makeover a couple of years ago. I know very little about wheels, but I do know that they have to sit properly on the hubs and everything has to fit. There are spacers, but...????
I would go to a reputable tyre fitter and speak to them, that is if you don't get a comprehensive response here. Year and model matter here, and tyre spec. Good luck.