Eagle Change Isolation of all Polygons
I personally do not know a way of globally changing polygon's isolation but a method I would use to make it quicker is to type in the command bar: 'Change isolate '0.35mm'' and then you can easily click on each polygon and it is much quicker than manually right clicking and changing properties.

I have just found out that the isolate parameter is only applicable to polygons. So to do it more quickly you can 'group' your entire PCB board and using the same command as above (or going through the tool bar menu) right-click and select change group - this will apply the change to all of your polygons without having to click many times!