It is possible, that's exactly what I did.

The only differences on the 64GB and 256GB models compared to the 512GB model are that the screen doesn’t have the anti-glare etching and you don’t get as nice of a case. Also you don’t get some software perks like special keyboard styles.

The anti glare etched glass is exclusive to the 512GB model - for whatever reason. I can't imagine it costing that much more to produce and I wish they'd have just put it on every model.

I wish they just made it an option to choose — I hate the matte effect, but I want the model with most storage.

If I used my Deck more than I actually do, I'd genuinely consider getting the 256GB model and upgrading the storage, just to get the glossy screen.

It's definitely not a matte screen, but a coating that cuts the worst of the glare. Kind of like a decent phone screen. Of course, I just wanted the storage, and then I put a glass screen protector on right away, making the glare even worse...

Here is a blog post that shows the differences most realistically to my perception of it.

https://www.louis993546.com/steam-deck-notes/

That’s what I was trying to say but lacked coffee. Fixed now.