The main aesthetic change is the homepage - as you can see in the middle frame above, the IGTV home screen will now feature a popular creator up at the top, in a sort of showcase frame. The creator highlighted to you will be based on your usage - and as you can see, the banner image is fairly prominent, which should encourage more viewing of top creator content.
That's an important element for IGTV - as many video platforms have found before, if you can't keep top creators around, you won't last for long. Vine eventually died out after its big names started to migrate to YouTube - and, interestingly, Instagram - while TikTok is now working to incorporate more monetization options for influencers in order to provide more revenue potential, and keep them posting.
Instagram's main app, of course, does hold significant appeal for top influencers, but IGTV itself hasn't yet taken off as a key platform. If Instagram can drive more traffic, and revenue potential, for creators via IGTV, that could play a big role in making it a more relevant option - and worth noting, Instagram also launched the first stage of IGTV monetization for top creators just last month.
In addition to the home page redesign, Instagram's also adding a new Discover tab (first frame in the top image set), providing a more specific way for users to navigate the app, and likely highlight top creators once again. Up till now, your IGTV search options have been limited, and largely defined by content you've already engaged with, but a new Explore option could help to boost discovery by giving users more specific control over what they see.
For reference, these are the current IGTV discovery options:
Instagram's also adding a new hands-free recording mode to provide new capture options:
"Tap the video icon on the top left corner to create and upload videos on IGTV instantly."
And it's also adding an important, cross-promotional update - now, when an IGTV creator posts a link to their IGTV video in their Instagram Story, the first 15 seconds of the video will play, with a 'swipe up' link to see the rest, as opposed to the current freeze-frame from the IGTV clip.
The changes, as noted, align with the shift towards the next stage for IGTV, in terms of monetization and expansion - though how useful they'll be is difficult to say, as Instagram hasn't reported how many people are actually using the separate IGTV app.