Contrast is followed by in

Removing the conjunction you get 3 sentences

We use actually to indicate a new topic of conversation

We use actually to indicate a change in what is being talked about

We use actually to indicate a contrast in what is being talked about

"Contrast in" is used in identifying where the contrast is. For instance, "Contrast in appearance"

The other versions are generally used to actively compare different instances.

This question is similar to a question recently asked; this answer is applicable here. The usual prepositions are "between", "with" and "to" and they introduce a noun complement. As "in" does not introduce a complement, it must be an adjunct. This adjunct, "in what is being talked about", tells where a contrast is found; it must be understood that it is found betwween the elements constituting what is being talked about.

I think it simply means that the part that follows actually in a sentence, will contain a new topic of conversation or a change or contrast compared to something that was asserted before actually. For example:

I was looking forward to reading your work on freedom of speech. Actually, the majority of the essay regards the question of women's emancipation.

The clause "the majority of the essay regards the question of women's emancipation" is introduced by actually, because there is a contrast IN it: between your work on freedom of speech and the majority of the essay regards the question of women's emancipation.

"IN" merely expresses where something is:

it expresses the situation of something that is or appears to be enclosed or surrounded by something else. (Oxford Languages)