Ok i know english is not my first language and maybe i dont undestand you well but in 0.5a, inside www folder there is an empty save folder so, if you owerwrite everything, you also overwrite the empty 05a save folder and the content of your old 0.5 save folder will be lost. I'm i wrong?
This is a computer realted term "merge"
When you copy a folder to an already existing folder with both having the same name , this process called "merging".
Folder name: www (old)
Folder content: image_1 + image_2
Folder name: www (new)
Folder content(files): *nothing*
The result will be as below:
Folder name: www
Folder content(files): image_1 + image_2
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The thing you should know the folder is getting merged, the files inside them will get replaced if it already exist with the same name.