I have added custom fields and sections to DocTypes (Quotation, Invoice, Sales Order), directly, without using the Customize Form. But new knowledge recommends making changes /additions via the Customize Form not directly, as this may cause conflicts when updating ERPNext.
SInce my site is already operational, how can I “move” these changes to Customize Form and leave the original doctype definitions?
Regards.
You can always stash your changes and re-apply after update.
Oh, okay. Thanks for the response. Is there anything I would do differently, considering I’m running under Docker?
Well i’m not really good with docker so i can’t really give you any helpful advise there.
From my experience, you better use the customization, not touch “mandatory” fields (they’re often very functional), and create a new app and export your customization as fixtures.
Since you’re using docker, you can spin up a new one and duplicate your changes bit cleaner 
Thanks for the suggestion. Yes I’ve realised that now. But as I have mentioned, this has been done already.
You should log into the server and give it a git status to see what files you’ve modded. You can stash and pop them, or save 'em into an archive and duplicate the changes on a different instance of ERPNext.