All the docs going back to 1.1 are at https://longhorn.io/docs/<version>. For instance, https://longhorn.io/docs/1.3.0/ for your current installation. Coverage of migration would be in the 1.4 docs, and the one think to stress is that upgrades shouldn't jump a version, so the recommended path is 1.3 to 1.4 before going to 1.5, if that's the plan.