skill substitution?

Consider the following:

Treat wounds p249

The Medicine check DC is usually 15, though the GM might
adjust it based on the circumstances, such as treating a patient
outside in a storm, or treating magically cursed wounds. If you’re
an expert in Medicine, you can instead attempt a DC 20 check
to increase the Hit Points regained by 10

Now, if I have

Natural Medicine p264
You can apply natural cures to heal your allies. You can use
Nature instead of Medicine to Treat Wounds.

...does this mean I can attempt a DC 20 Nature check to increase HP if I am Expert in Nature?

Question:

Do feats that enable skill substitution completely replace the original skill, or do you still require the original skill in certain circumstances (specialist use).

There are lots of feats like this. If you only read the substitution feat (Natural Medicine in this case) it feels quite straightforward - you can now forget about Medicine for purposes of Treat Medicine.

But if you read the Treat Wounds language with your rules lawyer glasses on, it can well be argued that you still require Expert in Medicine to be allowed to then use Nature to make that DC 20 check for extra HP.

What is the RAI here?

RAI, replace each instance of "Medicine" in Treat Wounds with "Nature". The feat is kinda painfully pointless unworthwhile otherwise.

RAI, replace each instance of "Medicine" in Treat Wounds with "Nature".

Do you have any source on this?

(Remember: I'm asking in general, only using this particular feat as an example)

Errr, sources are for RAW, not RAI.

If you want in general you'll have to cite a few more examples, as I don't remember anything else like Natural Medicine off the top of my head.

As the feat says, Yes.
You will be able to use nature instead of medicine for the treat wounds action.

If with expert medicine you will be able to roll for dc 20, so you will be with expert nature.

It's not really spelled out anywhere in the rules. Treat Wounds is the only skill action I can see in the rules that has this sort of if Expert you can do this thing. This sort of thing is fertile ground for an FAQ question.

Thank you!