I don't have enough information to estimate the seriousness of loosing this data (i.e., are there a few full-feature films, or hundreds of short takes; is it only a download issue, i.e., time and network bandwidth; ...).
But working on business software, we have this problem every quarter (release): Some data structures change, to support process or UX improvements. It is not an option to "corrupt" any data, as customers depend on our software for business continuity.
So the solution is to have a "migration process" for every such change: Take the existing customer data, turn it into the format required for the new version of the software, and only then upgrade the software.
For example in your case, create a list of all downloaded media, download them in the new format, then install the new app - which can immediately present all previously downloaded media.
From a UX point of view, that's what I would expect as a user. When any app upgrade destroys all my data, there's a huge risk that I will switch to another app - because I have to start over anyway. So this argument is not only about the user's experience (what we are discussing here), but also about the business consequence of a careless (considering technical process only) update procedure.