You can use the Import izard and reset deployment ID to migrate and upgrade LiveCycle 8.x assets into application model in Workbench ES2. Using the Import wizard, you can import LiveCycle 8.x assets one by one from the server into Workbench directly, such as processes, resources, events, etc. After resetting deployment ID, you can utilize Workbench ES2 or later new features.
The limitation of the method is that you can only import assets one at a time. In Workbench ES2.5, the Workbench team presented a new feature which is an alternative to having legacy resources migrated to the application model using the Archive Migration Tool. With this, it is presumed that you have an existing 8.x archive, LiveCycle Archive(LCA), which means you have to create an LiveCycle 8.x archive manually to collect all the LiveCycle 8.x assets you want to migrate and upgrade. The wizard will then create a new application, migrate all legacy assets, and upgrade them to make them Workbench ES2.5 compatible. You can download and use the Archive Migration tool from this link.