CaseBox

Modern and reliable text editors come with integrated case changing options, making it easy for you to edit your document's content. However, if you choose to use a more simple editor that does not include such functionality, a third party tool might be of great use if you want to quickly change the capitalization of a text. One example is CaseBox.

Minimalistic approach and a simple GUI

With a pretty suggestive name, this particular application aims to provide a quick and simple means to modify the case of a paragraph. It is portable, so no installation is required, which means you can just launch it with a double click.
The good thing about CaseBox is that it is simple. In other words, it only deploys a minimalistic window comprising a text area where you can paste your paragraph, alongside a few checkboxes to choose the capitalization mode and a few buttons to paste, clear and copy the text.

From upper to lower case and viceversa

The standard copy, paste and cut shotcut commands work, so you can just press CTRL-V in the text box to have it filled with your text section. Please note that large texts are not to be pasted in CaseBox, since this might result in application crashes.
CaseBox can convert the case from upper to lower, as well as capitalize each word in a sentence if the 'Title Case' mode is selected.

A sole purpose: to change the case of a small text

There's nothing much to say about CaseBox. Its straightforward approach makes it easy to use by anyone who needs a quick way to change the case of a small text.
However, you can surely perform the same using a more advanced text editor or, as an alternative, addons suited to this purpose.