If you just want to migrate the databases you can use Express Migration Tool 3.1, which can help you to migrate you data from version 8.1 to 9.0 Initial Release. While running the tool just select the option below:
Since the express migration tool does not support any version above 9.0 Initial release as a targeted version. Hence once you are there then you are run the SQL scripts mentioned in the update guide from 9.0 to 9.1 Update 1.