Been reading the policies and it says new apps and updates, I do not see anything about existing apps so I take it existing apps will be ok?
“Beginning August 1st 2014, all updates and new apps uploaded to the Play Store must use the advertising ID (when available on a device) in lieu of any other device identifiers for any advertising purposes.”
You answered to your question by writing “all updates and new apps uploaded to the Play Store must use the advertising ID” 
So existing apps are ok;)
But this is just when the Advertising ID is used for advertising purposes thats right? For example StartAPP SDK dont use this I think.
The way I see what they are saying is if you track the devices/users for advertising (targetting) you have to use the new id’s. If the advertising network doesn’t track the users why would they need to use the id’s. So I agree that some networks who don’t track the users don’t need to update.
New admob does not use it. I updated new sdk of new admob and google play service, but i dont see advertiser id anywhere
Advertising ID is used for interest based ads. This means that any developer or network should only target users based on previous behaviour by this ID. This introduction follows introduction of IDFA in iOS and it transfers the power to the user to opt out at any moment for any interest based advertising. In simple words this is the replication of the cookie model on mobile. Any ads that are delivered in a blind way without being based on previous actions of the users can exist without this ID. The enforcement from Google will start on 1st of August, however we do not know how strict and immediate will be this enforcement (Apple released it more than a year ago but started to enforce it only a couple of months ago). Just my 2 cents in this conversation, hope it helps