I have a Pentax K20D and a Pentax K100D. I want to create a DNG profile in ACR and LR for the K100D. I wanted to use the embeded K20D profile for a starting point but when I export the profile with a K100D label I can only see the profile when I open a K20D PEF. I cannot see it when I have a K100D PEF open. How do I change the target camera?
You don't and you can't.
Start from scratch with a color target shot by your K20D.
If true it seems like real shortcoming of the software. Starting from scratch wastes all the effort that went into the the other profile. ACR has been notoriously difficult to get the "Pentax Look" from with the lousy reds and weak greens it tends to produce.
I have a *istD and a K20D myself and see none of the problems you do.
Your expectation of being able to swap DNG profiles among camera models is unrealistic and irrational from my perspective.