The question would be improved if the asker stated the context: “In the following sentence, does the meaning change if the word позаяк is replaced by так як?”
But “позаяк is a (near-)synonym of так як. What nuances are there between the two words?” is also an acceptable question. Unless they are extremely common words with multiple meanings (e.g. prepositions), this is ok. It may or may not be a great question, it depends how much information can be found in dictionaries. Dictionaries are generally good at giving the general meaning, but poor at expressing fine nuances. Bilingual dictionaries are especially bad at distinguishing near-synonyms, so questions of this form are pretty common coming from non-natives who aren't very fluent.
(Background: I don't speak Ukrainian, but I'm a regular and moderator of French Language Stack Exchange.)