convert all paragraphs using a style

How can I convert all paragraphs using a style to another style?

I have a Document using "Default" style with justification set.
I want to change all to plain left justified.

I have created a new Style with the desired features.

The short answer to your question is: Select each block and apply the new style.

But once we are talking about more than a few blocks of text, it's easier to change the current style than to apply a new style.

To do that:

When you have the format sidebar open and you make format changes to a defined style (in your case "Default"), the name of the style in the dropdown menu at the top gets an asterisk ("*") and an "Update" button.

When you click that button, the style gets updated and through that all the elements that use that style get updated as well.

If you want to discard the style change instead, just select the name of the style again in the dropdown.