Advice: taking over a Group Ex class

I have been approached to teach 2 classes each week as a result of an instructor leaving the facility and I've accepted and will be teaching starting next week. This instructor who left was with the facility for many years and is well-liked by the members who attend these classes. I anticipate some disappointed faces and - possibly - some lower attendance. None of them know the instructor has left and they will be finding out from me. Any advice on how to handle this in general / how to introduce myself proactively at the start of the first few classes?