Excel Files Continuing to Open in Preview Even After Using Change All

Not sure why that is happening.

Possible Fixes for El Capitan or Later

You should try each, one at a time, then test to see if the problem is fixed before going on to the next.

Be sure to backup your files before proceeding if possible.

1. Resetting your Mac’s PRAM and NVRAM
2. Start the computer in Safe Mode, then restart normally. This is slower than a standard startup.
3. Repair the disk by booting the from the Recovery HD. Immediately after the chime hold down the Command and R keys until the Utility Menu appears. Choose Disk Utility and click on the Continue button. Select the indented (usually, Macintosh HD) volumeentry from the side list. Click on the First Aid button in the toolbar. Wait for the Done button to appear. Quit Disk Utility and returnto the Utility Menu. Restart the computer from the Apple Menu.
4. Create a New User Account Open Users & Groups preferences. Click on the lock icon and enter your Admin password when prompted. On the left under Current User click on the Add [+] button below Login Options. Setup a new Admin user account. Upon completion log out of your current account then log into the new account. If your problems cease, then consider switching to the new account and transferring your files to it - Transferring files from one User Account to another.
5. Install Combo Updater Download macOS Sierra 10.12.5 Combo Update and install.

Select an Excel file. Press Command-I to open the Get Info window. In the Open With panel select Excel from the drop down menu then click the Change All button.