Requested feature overview description.
I'd like to suggest to implement a Javascript script or similar that could convert the urls in the upload form into the higher version as documented in howto:sites and sources.

There is a project that already does that (Image Max URL) and it is open source, maybe it could be implemented on e621 or can be used as a base for a new script.

Why would it be useful?
It could solve one of the problems of uploading images when a higher quality version is available. It still doesn't resolve everything (for when a higher quality version available in another website, for example), but can still be helpful in some cases to avoid unnecessary workload for the staff.

What part(s) of the site page(s) are affected?
The upload page.

Having the URL change automatically could work, but I think it would be useful for new users just to have a link to howto:sites and sources with the other links and warnings on the Upload page, with a message like "If you are uploading an image from another website, please use the highest resolution file publically available: [link]"

Semi under construction with other stuff: forum #256093
Altough with current plan it will check the url, then notifies to check it if it's for a sample.