when changing WC12,0 from HTTP to HTTPS/SSL what are the necessary steps for Creo Parametric clients

Since step 1 of our (even ootb deployment) of Windchill has always been configure for SSL before using the tool... I don't know that I can offer advice on the impact of switching from http to https... however I would suggest having a test WC environment that matches your production WC environment to test these things before implementing in production. Even well documented configs can have unexpected issues and require testing.

Regular upload/checkin (at least once every day) is a good habit to get into regardless, but to my knowledge it should not be necessary for a switch to secure protocol.. @jbailey has a very good point in any regard that any change to be rolled out onto your Production environment should be tested on a Test environment first. You should not need to set up new caches, but you may need to update your link to the server unless you set up for http rewrite (see https://www.ptc.com/en/support/article/CS57803 and https://www.ptc.com/en/support/article/CS349437).