Not a real upgrade but rather a fix for the design flaw that only the Y belts are adjustable without disassembly - and nothing for the faint-hearted:
- home your printer
- put a mark onto the window on each side where the screw for the belt is
- screw off the windows. On the left side, you also need to screw off the aluminium cover above the screen (mark the position of the screw there as well)
- drill a 12mm hole into the window. A step drill is highly recommended since it is less aggressive that a normal drill. Use a file (or a milling machine if you have one) to remove the small part of the aluminum cover element that covers the hole you just made.
- assemble everything back together
Now you can access the X axis belt tensioners with a small 1/4" hex nut without needing do remove the side windows whenever the printer is homed.
(sorry, still no pictures here - I keep getting an error, regardless if I use my computer or my phone to upload them 
)
If you want, print plugs for the holes, but I think they should be unnecessary - the gaps in the doors are much larger.
Edit: thanks to i3sven, you can find the pictures further down in this thread. Direct link: Snapmaker J1/ --- Upgrades--- - #10 by i3sven