The simple answer to this is yes. Should they - No. Are they potentially in breach of copyright with Thomas Telford licencing arrangements if they change it without highlighting - Yes. However, this does not mean that if they haven’t told you they have changed something and you didn’t notice at tender stage that you can rely on that as a defense.
You have to read your “terms and conditions” which is what a contract is at tender stage. What ever the rules are that they have changed is what you are signing up to. Most clients will highlight what has been amended - but often only by putting a Z prefix against a clause - you still have to read the original one to see what has changed.