As far as I can tell, there is no scenario at the moment except installing a new proxy.
My previous attempts ended in complete disaster, i.e. the proxy was not usable anymore and did not deliver content.
See my thread Katello 4.0 content proxy upgrade disaster
The docs for the content proxy upgrade from 2.3 to 2.4 are completely oblivious of the pulp2 to pulp3 migration: Upgrading and Updating Foreman server