Suggestions on how to handle the currency change from Euro to Swiss Franc and back on the GAS tour?
Spend the EUR in Germany & Austria, spend the CHF in Switzerland.
Use credit cards to the maximum extent in Switzerland where they are widely acceptable minimizing the need for CHF cash. (Obtain CHF from bank ATMs). The Coop grocery stores, restaurants/cafes and shops are happy to have your business and accept the usual credit cards. (I use my Capital One Visa). On your last day(s) in Switzerland, juggle cash vs CC expenditures to spend down you CHF. You can burn off your excess CHF cash buying Swiss chocolates.
Keep them seperated and don't do any currency changing. Have Euros for Germany and Austria, have Swiss Francs for Switzerland. You can get what you need there on the spot from ATMs. And as the other poster mentioned, credit cards work very well for most purchases in all 3 countries.
We only got out about 60 CHF from an atm on the way into Switzerland, mainly because we knew we were going to make the optional ride up to Schilthorn. While that would not have been enough to purchase tickets for the gondola it didn't matter, because the guide initially paid for everyone's tickets who were present to go up. We then all repaid him a few days later with combinations of of swiss francs and euros. I imagine you could cc your tickets if your guide doesn't do that. Our guide also allowed those with leftover CHF to trade him for euros.
Your guide is going to be able to advise you on what is best for you to do based on your plans while you are there.
Changing physical Euro notes to CHF or vice-versa is usually possible at the road border or main stations, but do not expect a good rate (loose 10-15%), they have to make a profit.
The big supermarket chains (Co-op, Migros) take Euro (notes only, no coins), give change in Franks. This is usually a good rate which is programmed into the till, changes daily. Same for SBB (ticket counters AND all ticket machines).
Other shops and restaurants near the border or in tourist areas will often take Euros, smaller establishments have less good rates, rounded in their favour.
You will always need some CHF (beer, ice cream). keep the two currencies separate, to avoid confusion.
My guess is that your first stop in Switzerland will be at a rest stop. There will be ATMs where you can withdraw francs. There was an ATM in Lauterbrunnen, there must be at least one in Murren. When my RS tour left Lauterbrunnen for France, we knew in advance that we would have a rest stop in Switzerland. Most of us spent our remaining francs there (and some euros too) on souvenirs, food, drink and especially Swiss chocolate. The chocolate sold in Switzerland is better than that exported by the same brands.
I took the tour several years ago. Our guide indicated we could exchange our leftover CHF for euros at a bank at our next stop in Munich. Like Chris said you take a bit of a hit but if you plan correctly you're only exchanging a small amount. In my case about 50 CHF. So the exchange costs were fairly minor.
We weren't on a tour -- got CHF at the train station in Lausanne as we
got off the train from Paris. When we left 4 days later we still had CHF so ... I guess we HAVE to go back. Sigh.