This is an old story and most of us who are in the offshore development world have been through it. Before you talk to your client, I would be sure to have 100% clear position on where the project is, if the project is actually poor or not, and whether it's a good idea to switch teams mid-project. These are huge decisions and although the client is ultimately in charge, it's up to you to give strong, confident advice about what to do.
What is your personal opinion on the team's progress? If they are doing OK and there is no chance that another team will do much better, then there is little to be gained by going through the pain of changing teams. If they are sluggish or burnt out, then so be it but that decision has to come from you first and only as a last resort should you agree to such a drastic move only because the client asks for it.
If you are to change, you need to do it quickly. You need to do it aggressively, and you need to make sure that the previous team provides all the assets and if possible they stay on for an overlap period. You can usually negotiate this with a good team if necessary.
If you really get into a jam, there are people like me who specialize in bail-outs of trouble projects.
Out of curiosity, what is your relationship with the team (i.e. are they a permanent team of yours or a first project, etc.), where are they located, and what language/environment is the app in?