Upgrading an Arch install months or even years out of date is not that big of a deal. That’s one of the benefits of a rolling release platform.
Once after a move, an old desktop sat in a box for at least two years and I had it updated in a hour or so. Yes, you have to review the archlinux.org news feed for breaking changes, but if you follow any steps that pertain to your packages it’ll work fine.
Well, updating can cause problems whenever you do it.
Technically, you should check the news feed for breaking changes whenever you update your system. Usually, the worst that happens is pacman just barfs. Then you can figure out why and apply any fixes.