duplicating my own roblox game to test a big update

Also don't forget version history. In the Creator Dashboard you can restore an older version of a place, so if the big update goes wrong on live you're not stuck. Doesn't touch datastores though, only the place file.

In Studio do File > Save to File to get a local .rbxl copy, open that, then Publish to Roblox As and create a new experience instead of overwriting. Keep the new one private and you've got your test copy. Two things catch people out. Datastores belong to each experience, so the copy starts with no player data (usually what you want, but your test save won't match live). And game pass and dev product IDs belong to the original, so purchase prompts in the copy still point at the live game's items. Swap the IDs if you're testing purchases.