We would like to travel from Switzerland to Bayeux in August. We already have Eurail passes, but it doesn't look like we can use them for the Paris to Bayeux leg of the trip. Do we need to book separate tickets through SNCF? If we have a delay, can we change the tickets? Should we get them online in advance or from the Paris St. Lazare station when we arrive. And should we book round trip to get back to Paris? Thank you!

Yes, you need to buy tickets for the leg to Bayeux. You can, with some conditions, change the ticket you've purchased BEFORE the departure time. Take a look here: https://www.sncf-connect.com/en-en/h...0full%20refund

IF you plan on returning to Paris, it is cheaper to book a round trip than two one-ways. Again, you can change dates before the departure so booking a ticket ahead, even if you don't know the exact time or date is possible.

I just took a rouind trip from St., Lazar to Bayeux. I bought the tickets through Rail Europe. It's quite east. I think, just like plane tickets, trane tickets purchased in advance are cheaper.