If you had/have an Office 365 tenant running on SharePoint Online 2010, you will be automatically upgraded to the latest version: SharePoint Online 2013.
Before this upgrade is proceeded, you will be notified and you will be allowed to evaluate the upgrade by selecting pilot users – or also postpone the upgrade process.
In case you start a pilot, you may noticed that existing SharePoint site collection may not be allowed for upgrade as shown below (this apply also for My Site)
Also, if you go to the SharePoint Administration portal, you will not have the ability to create new site collection using the 2013 template version;, even if the SharePoint Administration portal has been upgraded to 2013
This is due to the fact that upgrade features have been disabled by default.
To enable it and choose if you want to allow upgrade and 2013 site collection version creation, go to the SharePoint Administration portal and click on the Settings option from the left menu
Then at the Global Experience Version Settings section you will have the choice to allow upgrade/use 2013 version as well as if you want to maintain the choice to create 2010 version
- Allow creation of old version site collections, but prevent creation of new version site collections. Prevent opt-in upgrade to the new version site collections
- Allow creation of old version site collections, and creation of new version site collections. Allow opt-in upgrade to the new version site collections
- Prevent creation of old version site collections, but allow creation of new version site collections. Allow opt-in upgrade to the new version site collections
As soon as you allow 2013 site collection creation and upgrade, you will have any way the ability to disable the upgrade for specific site collection.
From the Site Collections section from the left menu, select the site collection you want to block the upgrade, click on the Upgrade\Site Collection Upgrade Settings button and set to No the option for Allow upgrade