Hi Kyle,
If you are using older version of angular then you don't have to worry about breaking functionality. But you do not get the new features of Angular , which is introduce in new version.
And if in case you will plan to upgrade your angular version in future then it will create many issue , which take some time to fix.
For example you are using ang2 and you want to upgrade with ang9--------not easy task.
You can easily upgrade ang2 to ang4, but ang4 to ang5 you have to fix many issue.
So the better idea is that you have to upgrade your application as soon as angular releases a new version, instead of directly upgrade from ang4 to ang9.