"...are subject to change" means that they change. It doesn't necessarily mean they are likely to change, it simply tells you that they do change.
You are more likely to come across the phrase:
may be subject to change which means that they can change.
The phrase doesn't have to be used with the verb be (be verbs), but often is.
Your example is correct, and is a case where you do not need part of the verb be. Another example is:
Ticket prices increase subject to decreases in average ticket sales.