Question on Multi DB I need A Clear Answer

We all know we have the old Database we use for installing the Wordpress Multisite,

Now I try to Install 4096 DB scale

With a global DB plus the 4096 DB...

My Question is will the Old Database be usable still or not? or should i just edit the wp-config.php and use the wpmu_global Database

I try editing , and still functioning?

Has Anyone Tried it like i did?

Or is this the right way of doing it?