I'm using this add-on and can answer this question since the developer hasn't. Editing a Template will affects whatever blocks are using it with that template.

On the one hand, it's very easy to Copy and Create a New Template if that's not what you want. On the other hand,you can end up with a lot of different templates, which I think is a shortcoming of the add-on design (there's no organizational features, just naming of the templates).

Also, I suspect it's a good idea to create new templates because if you do change a template and make an error, you will create problems on pages using the block with that template that were working fine before. (And you may forget and find out later the page is broken).

The addon really is useful, and the title appropriate because it's QUICK to use. You can do the same things and more with Block Designer, but using Block Designer takes more time. It really isn't as Quick and Easy.

Sorry for the late response, I see the discussion today

@Michael: You have to create a new template for your new block look. Editing the template will cause other blocks which is using the same block.

@ IKDnet: Thank you very much for taking the time to give me that information!
@ manup: No problem, thank you. So, to resume, if i edit an existing template, this will affect all existing blocks using that template. Evidently, doing so would rarely be reasonable, unless one knows exactly what he's doing... It seems that in most cases one should create a new template for new requirements.

Thank you for helping me to understand how it works!

Best regards,
Michael