If there was a book series called something, can another book series be called the same thing, but with different letters?

Yes, this why the author's name is on the cover.

However, there are two points in which this would matter.

If you made any attempt to portray your series as part of or related to that other series. You can't sell your book as being connected to that series. You can sell Lord of the Rings by Barnaby Cubbins, as long as your version of Lord of the Rings isn't about elves and such, but is merely about a houeswife who cleans bath tubs. As long as you never deceive anyone to sell books, you are in the clear.

Secondly, you'd have difficulty getting merchandising rights. I can write a book called The Who, a lovable nature tale, but I can't sell a T shirt that says the Who on it.

If I were to write a fantasy series called "The Stronghold Fire", would people tell me that my title is too similar to "The Lord of the Rings"? The two contain exactly the same letters. Go ahead, look it up, I'll wait. What if I were to call my series "The God of the Bands"? I would be using entirely different letters but my title would be a blatant copy of Tolkien's. In my experience, if you find yourself asking "Is my work/title/character/plot, etc., too similar to this existing one?", the answer is usually, "Yes, it is."

I literally do not understand what you mean here. Can you give an example?

Yes, but it might be hell to get it published because of the similarities.