the describe link is "owned" by the WTpart. If you revise the WTPart, the link is carried forward by the Out of the Box business logic.
But if your revise the WTdoc only, you will have a standalone new version of your WTDoc. The link is not a Versionable object.
If you do not want to revise the WTpart, you have to checkout (not necessary by code) the WTPart. remove the link to the old version of the Doc, and link to the new version of your Doc.
If you want to revise also the WTpart. You should be able to use "collector API" (don't know exactly which API, but it exists), to revise both objects together, and the link will be carried forward between the 2 new versions.