Or you could display only vias, then use the Goto bar to select all the vias of that style. Choose Pad Style in the Goto bar, click on the name of the pad style, then right-click and do 'Select All Visible Find Items'. This should select all the vias of that style. Then you should be able to open the Properties and change the style of all of them at once.
I changed it the way edrees suggested and it did the job. But the suggestion from DavidM seems also interesting I wasn'nt aware o selecting all pads of certain style with the goto bar.
Remains one question for me. How do I display only via's ?
Or leave the layers on, and turn off the 'categories' for pads and symbol pads. You can do both (hide pads, and hide all layers) from the Layers dockable bar so you don't even need to open the Display dialog.