Adding:  Strictly speaking you can change within one PZ3, but you
have to do it manually.   Let's say you want to use Dolly for 1-100
and Main for 101-300.   Pick Dolly; render out frames 1-100.
Pick Main; render out frames 101-300.

This is not hard, but it's not really automatic and doesn't leave
any indication in the PZ3 of what really happened.... if you want
to keep the scene around for future reference, you'll have to make
a separate note of how to use it.

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