Уперше/вперше

[EvgenFirst]

I have a question, my friends.
I often see the alternation of the у/в letters in the begining of words:
уперше/вперше,
уперший/вперший.
And many more( Can't remember more examples right now, but reading books and using dictionaries I often see this alternation.
Is this some kind of different standards, archaic/modren rule, or something else?

[Myron1313]

See Vinnfred's and mizail's comments.

https://www.duolingo.com/comment/8950518

There may be more comments elsewhere, I don't know.

[Vinnfred]

It's also in this post: https://www.duolingo.com/comment/9012790

[Ganya-ganya]

http://slovopedia.org.ua/32/53394/30391.html
Это, наверно вопрос мелодичности, больше.