I have been saving svg files using the Save a Copy. But when I re-open the file and go to Undo History, there is no history to undo, even though I made a lot of changes to the file before I saved and closed it. All I can see is: [Unchanged]. Opening the layers is OK, I can do that, but if I want to undo history (eg. undoing Union), I am not able to do it.
Is there a way of restoring the full history so that I can see to the beginning when the first changes were made?
That's not possible: history is not saved with the other work in the SVG file. I guess the best you can do is save each possibly-interesting version in a different file.
SVG files being "plain text", there are lots of tools to work with their content! You can use git to track changes on a file (or set of files) keeping track of every version (and changes between them) as you develop your image, sharing content with others and allowing them to make changes, keeping track of who made what changes, and keeping notes on every change that was made.
Unfortunately, I don't know of any way to persist a less-formal version control strategy like the undo history.
