Change to Third Party part policy

[Eugen]

While CC BY 4.0 allows for modifications, per library policy, all attributed authors must agree to the Contributor's Agreement.

No, only submitter: https://www.ldraw.org/article/349.html  

[...] the submitter ("the Author"), agrees to release the Work under the Creative Commons Attribution License 4.0 International License ("CC BY 4.0")

As derivated work from CC BY 4.0 work is a new CC BY 4.0 original work, previous authors could not be sumitters, because its no more their work.

The only policy here should be to follow that derivated work should not be idential bit-in-bit — there should be some changes inderivated work to claim it derivated.