What happens to my 2nd wife if I stop being polygamous in CK3?

I just tested this as an Ash'ari (Muslim) ruler converting to Catholicism.

This is what happened to my 3 secondary wives:

• I received 3 messages which read "Spouse Invalid - Your new Faith's beliefs mean that [name] is no longer your spouse".
• They became regular courtiers in my court.
• They did not change their religions (but my primary wife didn't either)
• They did not have an opinion penalty for being divorced (just one for me being an Evil [sic] Catholic).
• A couple days later, one of my ex-wives informed me that she is pregnant and that "Since we were married when this child was conceived, there is no stain of bastardy; when the babe is born it will be known as your legitimate offspring".

So TL;DR: Converting to a monogamous religion will get you a "free" divorce from your secondary wives without any of the usual restrictions, costs or penalties.