I understand that は and が can be very different in meaning & emotion, and some
structures require one of them. However, are there some cases where I can use
either of them but the sentence remains (almost) the same in meaning &
emotion?

(That is to say, given the same context, a native speaker would choose to use
either of them to express the same idea.)
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grammar particle-は particle-が は-and-が
Are there some cases where は and が are interchangeable?
Actually, yes.

In negative sentences は often replaces が or を. E.g.

本 **は** 読まない (hon wa yomanai) I don't read books.

彼 **は** 好きじゃない (kare wa suki jya nai) I don't love/like him.

In these sentences, it would also be grammatically correct to use を and が
respectfully, but many Japanese find は to be more natural. The rule of thumb
is that there needs to be at least one は in negative sentences. If you add 私は
at the beginning of the sentences above, the need for は is satisfied,
therefore speakers are less compelled to use は instead of が or を. In negative
sentences it may even follow particles like に or で to satisfy this condition.

英語 **では** 話したくないです. (eigo de wa hanashitakunai desu) I don't want to talk in
English.

I hope this helps.
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This discussion is probably as old as learning japanese is, and as the comment
mentioned, is pretty challenging to answer. You can find heaps and heaps of
literature on the internet about the differences between these two particles.

I can't really think of a case where they don't change the meaning though.
When は replaces が, it indicates a subject that is also known to the speaker,
or a contrastive element, or indicates that the sentence is now about the
topic instead of what follows. Let's try some examples to try to illustrate
this:

私は知っている "I know"

私が知っている " _I_ know"

At face value these sentences look the same. The が version makes it sound like
the focus is on you, that is, you are the one who knows (likely, you are
correcting someone).

The は version is more neutral because it simply establishes you as the topic,
and 知っている is stating something about you.

When が appears for transitive adjectives like 好き, or ほしい, it marks the object
that the adjective applies to. Specifically:

私は犬が好き

is natural. It simply says that I like dogs. However:

私が犬は好き

is... weird. It sounds like you like dogs, but not something else. This is
because here, は marks a contrast with something unnamed. Transitive adjectives
almost always mark the object with が for this reason.

The answer is... no. There's always a difference. When が is used to mark
something in the main clause, it usually places emphasis on it, as in the
first example. When は is used for transitive adjectives or inside relative
clauses, or as a second は in a clause, it marks a contrastive element, so it
is usually understood to mean "this, but not something else".

References: A Dictionary of Basic Japanese Grammar
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