April,
We do not support an "Update via Gedcom reupload".
There will be 3 ways to create a CET.
1) 3rd Party Integration (Preferred): Create & populate a CET from within a 3rd party software product integrated with the FamilySearch CET system. Such products will be able to create a CET, populate it with all the data from the desktop/web, and automatically keep the CET/external tree in sync as changes are made in either one. All of the CET populating and syncing can happen with very little user interaction. FamilySearch has been working closely with all the major vendors and we expect the leading software product to support CETs in the near future.
2) Gedcom Upload (not-preferred, but for early access): When a CET is created via Gedcom upload it is not tied to any external software product and the only way to edit the tree is online at FamilySearch. This option is intended primarily for users whose software product has not integrated with FamilySearch. It is the first of the 3 options to be delivered and the only one available currently to facilitate early access testing.
3) From Family Tree: This option will be delivered at a future date and will likely create a CET and populate it will a few generations of data from Family Tree (4gen, 5gen?) starting with the user. It is intended for those users who, rather than keeping their best tree data in an external product, make use of the Family Tree as their primary genealogy software, but need a CET.

To answer your question, What does this mean for your testing? During testing, you will probably want to manually update your desktop product with any conclusions and data that you add to your CET. You may want to contact your software provider and inquire about their planned support for FamilySearch CETs (we will let all vendors know when a final feature name has been determined, and we obviously cannot preannounce any 3rd party companies development plans.) When your vendor releases support for CETs, you may need to delete your tree and re-create with from within their product. We are exploring a way for a user to make a designation on a Gedcom created CET that will associate it with specific software product, but I can't guarantee the possibility, function or timing on such a feature.

Robert Kehrer, sr product manager