Alright folks - I thought I'd share my struggles / lessons learned for the early adopters of the Legion GO on how to upgrade your SSD.

Create a USB Recovery Drive First! Unlike the ROG Ally, there's no cloud recovery feature. You'll need to go to the below Legion Support URL and download relevant recovery media. Have your device serial number at the ready, and make sure you create a Lenovo account. https://pcsupport.lenovo.com/us/en/products/laptops-and-netbooks/legion-series/legion-go-8apu1/?linkTrack=Caps%3ABody_BrowseProduct
Bonus Lesson Learned: Do this on a Windows PC. The Recovery media creation utility only appears to be available as an .exe.
Have a Dongle on hand - It's useful to have a USB hub on hand to help with the reinstall. I used a keyboard to help navigate the boot drive menu.
RTFM! Read the Friggin Manual --- Lenovo has a useful PDF with a section on how to open up the Legion GO and remove the SSD. https://download.lenovo.com/consumer/mobiles_pub/legion_go_hmm.pdf. It takes a bit more force / effort to open up the device compared to the steam deck and Ally, but the plastic is pretty durable.
Be Careful with the Mylar. The SSD is covered in a mylar sleeve -- unlike the Steam Deck's SSD, you can't easily slide out the drive. It's best to unwrap it like a stick of bubble gum, but very carefully -- it's very fragile!
Don't over tighten the screws when reassembling the device! Luckily I didn't strip my screws or damage the device, but I've done that on too many other occasions... don't over-tighten them.
Be patient - Related to the above... just take your time with the process!