It depends how much older.
Like just a few days, weeks or continue to use a older no longer maintained version of a package because you don't like the new version.
I (now) guess you mean the later in which case, yes it's not trivial as it's not really supported by anyone anywhere. Neither the original software developers (which replaced it with a newer version), the package maintainers/Linux distro (which also moved on as the older version doesn't get updates), other packages interacting with ti (which expect a newer version), etc.
So while it might suck, I would recommend to simply not do so as it's not worth the risk and headaches it brings you. At least not as long as you don't find enough people to do a fork and maintain that fork.
Still undoing you last update is often as simple as just installing the package from the cache, and then you would need to pin it/make pacman ignore it (and then it probably will brake sooner or later depending how much it relies on specific system libraries in specific versions being available).