Стоит только заменить оперение или хвостовик, как дротик летит иначе.

This is a common Russian pattern meaning:

• one has only to ..., and ...
• as soon as ..., ... happens
• it only takes ... for ... to happen

So Стоит только заменить оперение или хвостовик, как дротик летит иначе means something like:

• You only have to replace the fletching or the tailpiece, and the dart flies differently.
• As soon as the fletching or tailpiece is replaced, the dart flies differently.

It expresses an immediate or automatic consequence.

Here it is from стоить, not стоять.

That can confuse learners, because стоит can mean either:

• costs / is worth / it takes ← from стоить
• stands / is standing ← from стоять

In this sentence, because стоит is followed by только + infinitive (заменить), it is the idiomatic construction from стоить:

• стоит только сделать X, как Y... = one only has to do X, and Y...

So it does not mean stands here.

Russian often leaves the subject unstated when the meaning is general.

In English, we often say:

• if you replace...
• if someone replaces...
• if one replaces...

Russian can simply use the infinitive:

• Стоит только заменить...

The doer is understood in a general sense: you / someone / one. The sentence is about what happens whenever that action is done, not about a specific person.

Заменить is perfective, and that fits the idea here very well.

Perfective is used because the sentence refers to a completed change:

• first, you replace the part
• then, as a result, the dart flies differently

The action is seen as a single whole event. That is why заменить sounds natural.

If you used заменять (imperfective), it would suggest an ongoing or habitual process, which is less suitable in this construction.

So:

• стоит только заменить... = once the replacement is made / you only have to replace...
• not so much: while replacing or repeatedly replacing

They are in the accusative case, because they are the direct objects of заменить.

You replace what?

• оперение
• хвостовик

However, both words look the same as their dictionary forms, which may make the case hard to notice.

Why?

• оперение is neuter inanimate
• хвостовик is masculine inanimate

For many inanimate nouns, the accusative singular is the same as the nominative singular. So the forms do not change here.

No. Here как does not mean how.

In this construction, как introduces the result clause:

• Стоит только X, как Y
• one only has to do X, and then Y happens

So here как is closer to:

• and
• and then
• when
• as soon as

This is a special fixed pattern, so it should be understood as part of the whole expression, not translated word-for-word.

Because как дротик летит иначе is a separate clause.

The sentence has two parts:

1. Стоит только заменить оперение или хвостовик
2. как дротик летит иначе

Russian normally separates such clauses with a comma. In this pattern, the comma before как is standard.

Because the sentence describes a general fact or regular result.

Russian often uses the present tense for things that happen characteristically:

• whenever this change is made, the dart flies differently

So летит here means something like:

• flies
• will fly, in such circumstances
• tends to fly

If you wanted to talk about one specific future event, you might use полетит instead:

• ...как дротик полетит иначе = ...and then the dart will fly differently

But летит is very natural for stating a general principle.

Иначе is an adverb meaning:

• differently
• otherwise

So дротик летит иначе = the dart flies differently.

It comes at the end because that position gives it emphasis. The important new information is how the dart flies: differently.

Russian word order is flexible, but this order sounds natural and highlights the result.

Sometimes yes, but it changes the feel.

• Стоит только заменить..., как... = you only have to replace..., and...
• Стоит заменить..., как... = still possible, but less emphatic

Только strengthens the idea that a small single action is enough to produce the result. It makes the cause-and-effect relationship sound sharper.

Yes. A few common alternatives are:

• Достаточно заменить оперение или хвостовик, и дротик летит иначе.
  = It is enough to replace the fletching or tailpiece, and the dart flies differently.

• Стоит лишь заменить оперение или хвостовик, как дротик летит иначе.
  Лишь is similar to только here.

• Как только заменишь оперение или хвостовик, дротик полетит иначе.
  = As soon as you replace the fletching or tailpiece, the dart will fly differently.

These versions are close in meaning, but the original sentence is a very standard and elegant way to express all it takes is X, and Y happens.