Forms and Overwriting only Cells

I have a form for Safety Credentials which feeds into my Source Sheet.

When the user fills out the form, they may only put in one credential.

If the user previously put in a credential it creates a new row and updates the pertinent cells in a Destination sheet. All of this works great.

My concern is that there are only 20,000 rows allowed in the Source Sheet.

If a user row is kicked off because the limit is reached, and the user row has a still valid credential in one of the cells, then that credential will disappear in the Destination sheet.

Is there any way for me to combine new cell information from the form with existing users?

I am attaching the Source Sheet as a screenshot. You can see the Safety Credentials that are filled in with the form. But one user may fill out only one credential each time.