I've finally come around to updating to org version 9.5. A change that I'm a fan of is the default for org-adapt-indentation is now nil since 9.5. However, my primary org file has been written with the hard indentation from org-adapt-indentation t. Is there an easy way to logically remove the preceding whitespace from this? I've tried using delete-whitespace-rectangle, but this doesn't preserve the indentation of subsetted bullets.

Is there any way to essentially shift the hard tabbed indentation to the left to remove the spaces but preserve the structure?

EDIT: I used two different strategies for this. I think the best strategy is to use org-unindent-buffer as recommended by blablook below. However, in my case, using indent-rigidly to shift everything to the left worked for me as well. Again, org-unindent-buffer seems to be the best.