...

If you have previously only enabled the Email Threading and none of the sub options Use References Header checkboxand Use In-Reply-To Header then both options will be enabled. Conversely if you had Email Threading disabled then the two aforementioned options will now be set to disabled.

What do I need to do?

There are no changes required on your end as this will be automated. If you do not wish for both options to be enabled, you will need to manually change and disable the association headers you do not wish to use.

...