Adding brackets (or parentheses) to end character

I have a series of files that cycle end letters (A, B, C, D, ect.) and then start over again on the next set of files. I would like to be able to add brackets [ ] around the letters and also make the letters lower case. How can this be done?

What have you done so far?
And why didn't you do it when you applied this procedure?

Why do you not use this approach?

If you show us some real examples, this would help to make suggestions.