Back in December we announced the launch of the new Google Sheets for Rapid Release domains. This new version is faster and can support larger spreadsheets.
Over the next couple of weeks, Scheduled release domain users will automatically be upgraded to the new Sheets. Spreadsheets created after the upgrade will use the new version.
We will begin upgrading Scheduled Release Track domains starting on April 21st 2014 on a slow roll-out taking place over 3-4 days. It may take a few days before it's available in your domain.
This upgrade does not affect existing spreadsheets and only relates to any new spreadsheets created. The user will not see any prompts about the new Sheets unless they create a new spreadsheet. Feature differences between the old and new sheets are explained in this support article
Google Apps for Business, Education and Government