It seems there is no way to link up, only to link down.

Do I have this right?

Now, having done that in a part, how do I link that Part's Assignable Knob to the Performance's Assignable Knob 1.

From the Part’s Control Assign screen (where you Assigned things to the Part 1 Assign Knob 1) touch the shortcut box “Edit Common Control Settings”

This takes you directly to the Performance Common/Audio level of the architecture, where you make the link to the Performance’s Assign Knobs.
The “Source” = AsgnKnob 1
Tap the “+” to add the Common Assign Knob “Destination” to control the Part Assign Knob
When you tap the “+” a default “Destination” parameter is shown... tap that default parameter so you can see the pop-in menu with all of your Assign options.

Select “Part 1” > “Part 1 Assign 1” as the “Destination”
Press [ENTER]

You have now assigned the Common Assign Knob to control the Part 1, AssignKnob 1.

By default, the Common Assign Knobs are linked to the Super Knob movement.

Thank you. I had always presumed that button was redundant, due to the naming of it.

It should be called:

Edit Performance Common Control Settings

If that doesn't fit:

Edit Common
Performance
Ctrl Settings