I believe it's possible to run a migration script as an "append" instead of a "nuke and replace".
You'll have to ask @akhoury more about that, unfortunately 😦
@Ganzuelo The uploader is the right way of doing it (if the file is on your computer, but you have to wait until it changes from 100% to the imgur link) if you already have a URL, then you use the code in my previous post. Julian is working on changing the image button slightly to allow you to enter a URL manually without needing to know markdown.
Alternatively, the imgbed plugin would allow you to just enter the link and it auto embeds. Just not enabled here I don't think.