If you just want to migrate the databases you can use Express Migration Tool 3.1, which can help you to migrate youthe 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 thenafter express migration you areneed to run the SQL scripts mentioned in the updateupgrade guide from 9.0 to 9.1 Update 1.