I am not aware of any tool which supports seeding updates for an existing release, but yambs allows you to do it and it works fine for most fields.

When I attempted to POST a release seed which contains labels, I failed to make it work in a meaningful way. If we assume that the target release currently has label A, there are three basic use cases:

1. Replace label A by label B: Seeding only label B gives the desired result.
2. Remove label A: Apparently not possible, seeding no label keeps the existing data intact.
3. Keep label A and add label B: Seeding label A and label B seems to be the obvious choice, but this causes the edit preview to fail silently with HTTP 400: {"error":"The label and catalog number in this edit already exist on the release."}

While I can live with the behavior in case 2, the last case is very annoying as the eror is neither displayed nor fixable by the user. Removing the visible "duplicate" label A willl enter an edit to remove it and there is no way to get the "original" label A back.

P.S. I had expected external links to have the same issue, but for these the seeded ones are always added in addition to the existing ones.