It's a troublesome process. Even changing job is more fuss free. But the easier part in changing project in ACN is you shouldn't have issue passing the interview... lol.

There's no proper process to go about it...

You either request for roll off and look for projects or
You inform your project you don't want to be extended and start looking for projs.

How to look for projects? It's all about connections, else you have to fallback on your TFS.
If you have no connections, then you have to ask yourself wtf have you been doing in ACN for the past few years. Yes, your PL, your PM, your MD are all connections, look for them. Most often than not if your PM is handling multiple projects or know some projects under the account is looking for people, they can help you switch.