How will and when will Mobile browsers start serving AMP (Accelerated Mobile Pages) Content?
Websites shouldn't switch their entire frontends to AMP. AMP should be used to accelerate static, article-based content when requested by the client.
In order to inform clients that an AMP version of the content is available you can use a link tag in the document's head like:
<link rel="amphtml" href="http://www.bbc.co.uk/news/amp/34471849">
When Google or another app indexes the page it becomes aware that there is an AMP version. The cached version of that AMP page will be served up when a user navigates to the page via a link or within a carousel of stories.
If you want to start viewing AMP versions now I've released a Chrome plugin which redirects you to the AMP version if it exists. Preferences are configurable per URL, per domain and there is a configurable default.
https://chrome.google.com/webstore/detail/amp-accelerated-mobile-pa/baodpdnidmedcpanneabeldenamaaeoe