As you’ve discovered, Part 1 can never be empty — that is why you cannot use “Performance Merge” on Part 1 (“Performance Merge” copies all Parts of the selected item in one operation).
Recommendation: Do not Delete Parts 2 and 3 of the “Imperial Grand”... rather Replace the original Parts - one by one. This will reconstruct the “CFX PopRock” Piano.

You are replacing Part 1, therefore, you can only select one Part at a time... but you simply need to repeat, the process... incrementing the “Source”. The “Source” setting lets you copy any single Part from any existing Performance

Replace Part 1 as you did... then select Part 2, but this time set the “Source” to CFX PopRock “Part 2”, then repeat again, select Part 3 and replace it by setting the “Source” to Part 3, finally, select Part 4 and Set the “Source” = Part 4
You are basically rebuilding the components one at a time

Alternate Method: Build a new Performance starting with the “CFX Pop/Rock”, then go get your Part 6 and 7 and by setting the Source appropriately move them (each by each) to this new Performance.
Hope that helps.

Thanks for quick response.
Seems a bit cumbersome.

As opposed to what? It is not cumbersome, by starting with the wrong piano you made it what it is...
When building a structure, say a building, the foundation is the most difficult thing to replace, replacing the roof is much easier.
Cumbersome? The request you make is cumbersome by its nature. It would be a lot less work to build the sound on the foundation you are going to keep. That’s logical, and would be much easier.

I think it's fair to point out that a multi-step solution is less desirable than a single-step solution. Performances are not always "born" out of mistakes. The scenario does point out a weakness or a lack of flexibility which requires more steps than selecting a "source" Performance and having it replace a contiguous number of PARTs in the "target" (current) Performance. The number of PARTs replaced being the number in the "source" Performance. And including being able to target PART 1 as the destination. I think that's a fair wish or expectation.

Also fair to say that can't be done - and here's what you have to do to get around it.

... and fair to think that it's "cumbersome" vs. the initial expectation.

Unlike some - I haven't seen Urs request to rewrite firmware or make threats to return the instrument or otherwise "fly off the handle". Seemed like healthy feedback was given - an honest opinion. I would rather see these opinions taken more seriously/respectfully. You can still outline that this isn't how the system works - but I'm not sure blame is the best way to go.

I guess I took the response to mean, ‘thanks but no thanks... is there an easier way.’ And not as constructive feedback. I offered an alternate method... that would be less work. Perhaps I should have just said, ‘you're welcome’, and ignored the critique.

For me, if you had to manually move the dozen or so Waveforms of one piano to replace the dozen or so Waveforms in the other piano. Or if you had to deal with the several hundred individual Samples on an individual basis... now, that would qualify as unwieldy. Or if you had to deal with controllers, Effects, and the hundreds of other settings.

The scenario does point out a weakness or a lack of flexibility which requires more steps than selecting a "source" Performance and having it replace a contiguous number of PARTs in the "target" (current) Performance. The number of PARTs replaced being the number in the "source" Performance. And including being able to target PART 1 as the destination.

When using “Performance Merge” you can place an entire Multi Part instrument in one operation. “Performance Merge” is the Search screen that appears when you are “+” adding a Single or Multi Part Performance to an existing Performance.

There will never be a “+” in Part 1... which is why you cannot “Merge” a Multi Part Performance starting at Part 1.
You must use the standard method of building a Performance, which, is what it is, one brick at a time. Not a “weakness” but how Performances are built. The ability to add multiple Parts is reserved for when you are “adding” to the current Performance... and not for “replacing” a component within the existing structure.

The other two Search methods “replace”.
“Performance Category Search” replaces the entire Performance, all 16 Parts
“Part Category Search” replaces the selected single Part

If your point is, ‘every product could be improved’, that’s always going to be true. I have to deal with how to get it done, now.
If and when the firmware changes, my function will still be how to get it done. I didn’t want to sound like I was angry or disrespectful, I was using my fun voice!

I think it's fair to point out that a multi-step solution is less desirable than a single-step solution. Performances are not always "born" out of mistakes. The scenario does point out a weakness or a lack of flexibility which requires more steps than selecting a "source" Performance and having it replace a contiguous number of PARTs in the "target" (current) Performance. The number of PARTs replaced being the number in the "source" Performance. And including being able to target PART 1 as the destination. I think that's a fair wish or expectation.

Also fair to say that can't be done - and here's what you have to do to get around it.

... and fair to think that it's "cumbersome" vs. the initial expectation.

Unlike some - I haven't seen Urs request to rewrite firmware or make threats to return the instrument or otherwise "fly off the handle". Seemed like healthy feedback was given - an honest opinion. I would rather see these opinions taken more seriously/respectfully. You can still outline that this isn't how the system works - but I'm not sure blame is the best way to go.

Thanks Jason, appreciate!