How do you label a sign with a score in Minecraft?

I once saw a YT video of someone "placing" a score on a sign. Now I've been doing research for a while, but also experimenting myself but to no avail.

How can you display a score on a sign?

The sign is placed with a right click, then you can type in a text.

If the sign is already there, the text can no longer be changed.

The answer from @VeryBestAnswers is complete nonsense. Of course it is possible to change signs afterwards, or to place them with a finished text. The latter is easier: Click on an existing sign with ctrl + middle mouse button and you have the sign. To change a text you have to use the / blockdata command.