You may want to switch to MathType.
The old Equation Editor was based on an abbreviated version of the commercial product MathType.
http://www.dessci.com/en/products/mathtype/

You can download the free 30-day trial and should be able to edit your legacy equations. If you let the trial lapse, you will be left with the equivalent of the old Equation Editor.


Here is a link to Microsoft's page on the new Equation Editor.
https://support.office.com/en-us/art...3-c7dea24fdf3f

Using MathType as a workaround is suggested on this Microsoft Page:
https://support.microsoft.com/en-us/...tion-in-office