Dec 3, 2014 - 7:26 AM
Nine minutes is plenty of time if everyone can carry their own luggage so you can go to the next train in one go without needing trolleys. Just don't dawdle and it should be fine. Platform numbers can be found in the timetable if you fill out your date and time of traveling. Last minute platform changes are possible, so do pay attention to the train intercom and info boards on the station.
From what I can see when filling out a random date in January both changes in Bern and Zurich are quite simple: in Bern you need to go the next platform using the escalators up or down via a tunnel. In Zurich it's two platforms onwards, either through escalators down or via the station hall at the same level.
The trip would be more comfortable without your luggage. The luggage service can ship it from Interlaken to Tirano for you. See here for information and links to more details.
By the way, my personal preference would be to stay the night in Chur and go there the day or evening before, but you've probably considered that option too.
Have a great trip!