Julia has reproducibility guarantee. Meaning if you have exact same versions you can run everything the same.
I.e. Julia 1.4.2, it needs not be JuliaPro, but it does need the same exact versions of your packages too. That’s what the Manifest.toml file is for.
[Julia is a system of its own in a way, and no upgrade of the OS has been known to break the guarantee that I know of. But in theory just upgrading the OS, and nothing else could do that, and that has happened with Python in the past. Also you WILL get a different stream of random numbers, which is usually not a problem, even better. Unless you only upgrade to, if I recall, Julia 1.7 or 1.6 LTS which is rather outdated. Bit-identical float results are hard to guarantee for any language upgrade, and you shouldn’t expect that, except for basic arithmetic operators.]
But in most cases it should be ok to upgrade (Julia has a forward compatibility syntax guarantee, and API stability guarantee, it’s just some package’s use undocumented, not-promised stable, (non) “API”.]. E.g. to latest supported Julia. People may tell you it’s not true, and it breaks the old packages. But if you also update the packages to latest then you’re usually fine.
If not, then a package depended on internals of old Julia, or internals of older package.
The Project file is often enough, it doesn’t state exact versions, then you would just upgrade to latest. To be sure you need the other (and would it alone be enough?).
And of course you need your own source code… And in some exceptional cases not all dependencies are handled by Julia, it’s package manager, and then you need to install something more.
You could still use the exact same Atom or newer, or VS Code. What I wrote about needing exact same versions for the guarantee would not apply to Atom, since it’s just an editor and to access Julia. It might still apply for familiarity. Note, the Atom people moved on to working on VS Code, so so should you, at least eventually. It should have feature parity, and lots of newer features. But it will be slightly different. I doubt anything you did relied on the exact IDE/editor(?).
I found some rather recent discussion related to Atom, but as I said at least the same version (and most probably the fastest should work):