Hi Albert,
I have not seen any cases in which users migrate backwards to the legacy
services, but theoretically, it should be possible. We recommend not using
both at the same time. As long as you're using one and are consistent, you
should be fine.
I suggest going over the Should I Migrate
>
> If you've only used feeds created by the AdWords user interface, you can
> use extension setting services immediately.
> However, if you've created custom feeds with the API, you'll need to first
> perform the following steps:
Allow me to confirm(double check), as long as I have been usin
