Simple; you just tell them that if they do not go ahead with it it might have negative repercussions in the future. Try to explain it in a positive and friendly manner. If after doing so they still do not change it, it is their own problem, you have done your part and they have simply refused to cooperate so if something goes wrong because of that it won't be your fault.
Now, of course, I do not know what are you getting them to change, but sometimes you need to understand that an established business won't be able to adapt to certain changes due to their brand guidelines.
Anyways, I believe if you approach them in a professional manner I am sure they will listen to you.