I don’t think you need any ‘unsafe’ code settings… maybe, but I’ve never heard of needing that for this type of upgrade.
DOTween may have an updated version, or better yet the actual source code from their github.
UnityEngine.UI is now provided via the package manager.
Spine I believe has been discontinued, but you might be able to work around it by googling the errors with that context.
I’ve done these sorts of upgrades many times. The basic steps are:
get the entire UNMODIFIED original project under source control (see below) before you change ANYTHING
work through each fix / upgrade / fiddle and test and commit incrementally so you can revert
if necessary, delete entire subsystems to get it to build sooner, then restore them from source control and fix them one at a time.
If you are not already doing so, please consider using proper industrial-grade enterprise-qualified source control in order to guard and protect your hard-earned work.
Personally I use git (completely outside of Unity) because it is free and there are tons of tutorials out there to help you set it up as well as free places to host your repo (BitBucket, Github, Gitlab, etc.).
You can also push git repositories to other drives: thumb drives, USB drives, network drives, etc., effectively putting a complete copy of the repository there.
As far as configuring Unity to play nice with git, keep this in mind:
I usually make a separate repository for each game, but I have some repositories with a bunch of smaller test games.
Here is how I use git in one of my games, Jetpack Kurt:
Using fine-grained source control as you work to refine your engineering:
Share/Sharing source code between projects:
Setting up an appropriate .gitignore file for Unity3D:
Generally setting Unity up (includes above .gitignore concepts):
It is only simple economics that you must expend as much effort into backing it up as you feel the work is worth in the first place. Digital storage is so unbelievably cheap today that you can buy gigabytes of flash drive storage for about the price of a cup of coffee. It’s simply ridiculous not to back up.
If you plan on joining the software industry, you will be required and expected to know how to use source control.
“Use source control or you will be really sad sooner or later.” - StarManta on the Unity3D forum boards