It's always best to pick the technologies you will need to use at the outset of a project, and usually hard to refactor or migrate your code to a different choice.
If I start a project with 'Bootstrap 3', and 'Bootstrap 4' comes out. I could migrate things over, but the real question would be: is refactoring your app worth the benefit of using the newer framework?
In this case I imagine choosing Angular1 is a much larger investment than a CSS framework is in terms of technical debt in your project, so unless you've hit a serious problem, or there's a major benefit to migrating to something newer, you shouldn't feel compelled to update or migrate.
What you are able to do is choose the newer Angular for newer projects, at their outset. And those projects will be Angular5 projects likely as long as they need to be maintained.
It's something we need to spend more time thinking about before we pick up tools and frameworks, because once you've chosen them, you're kind of married to them. A divorce from those original tools is possible, but messy and never a desirable option.