Can I change the email address that I used for my Bobclass Cloud account?

No. The email address that you initially used to create your Bobclass Cloud account is used as a user ID and cannot be changed. It is not used for communication with your clients. However, if you still want to use a different email address, you can simply create a new Bobclass Cloud account:

Go to the app's settings > Bobclass Cloud
Log out
On that same page, create a new Bobclass Cloud account
You will receive an email that you need to confirm.
Log in to the new account - this should be done from the device that contains the latest calendar/client data.
The app will now upload the entire database to cloud. This can take time and do not interrupt it! Leave the app in the foreground and make some tea.
Once the data is uploaded, go to your other devices where you log out from the old account and log in to the new account.