Back in December, we announced that Google would be releasing a new version of Sheets. Google will begin the roll-out of New Sheets to UMICH sometime over the coming weeks. This upgrade—which could take several days to complete—does not affect existing spreadsheets and only relates to any new spreadsheets created. You will not see any prompts about the new Sheets unless you create a new spreadsheet. Feature differences between the old and new sheets are explained in this support article. The release of New Sheets will also include Add-Ons for Sheets, which have already been activated in Docs. (See previous announcement.)
For more information, read Google's original announcement.