To what extent are differences in Unity versions acceptable?
Do they need to be exactly the same version?
I want to open a project created with ver2017.2.1p3.
Can I open with 2017.1.5f1 or 2017.2.5f1 or higher?
You should be fine to do it, in the past I've found that sometimes going jumping a full version (e.g 17-18) will cause the metadata to be cleared which detaches all objects from where they've been assigned through the inspector so just remember to take a backup.
My advice would be to step through the versions, try the highest 2017 version, then 2018, then the latest version, as long as you have a backup you can't really go wrong.