Can't update my account's non-Zoho email address

Can't update my account's non-Zoho email address

Years ago I set up a free Zoho mail account with a "Primary Mail ID" that no longer exists.
Later I set up a free Zoho account for a non-profit organization using my only existing non-Zoho email address as "Primary". 

How to I get Zoho Mail to allow both of these Zoho accounts to be associated with my only non-Zoho email address?  Is the only option to stop using Zoho and use another email service provider for one of these accounts?

I could possibly upgrade my personal email account to a paid Zoho Mail account, but that is not an option for the non-profit business' account.

Additional information:
When I try changing my "Primary Mail ID" to my Comcast email address I get the response "Email Id already registered".  I assume that is because I'm using it with the another Zoho account.  I need to use it for both accounts.