As mentioned by you, OpenUpgrade is a free script provided by OCA to migrate from different versions in Odoo community.
Though you might encounter different errors while executing the script, you can make necessary changes to the database accordingly and possibly achieve the migration.
There isn't a specific solution to the errors that you will face, as each time different errors might occur.
Also you'll have to make some changes to the script to make it run.
But on a short note its the best tool for migration.
Refer the below link for the blog on how to upgrade your database using OpenUpgrade.
On the second method, if your database data is not too large, you can create a fresh database and import the data's into it or using rpc methods like jsonrpc or xmlrpc to write python scripts to import data simultaneously from these versions.