I've just done a similar upgrade myself (10.4 to 2018.3) and it was a pretty simple process. Even with the introduction of Hyper, Tableau Server can still handle the old TDE data structure, so you don't have to worry too much about it. That said, when you refresh extracts, they'll automatically get upgraded. So, to do a full conversion, you just need to refresh your extracts. I wouldn't expect there to really be any challenges. It should be a pretty smooth process. But, as with any upgrade, be sure to do it in your UAT environment first, before moving to Production.