If the source and target database are the same platform, you have a couple of choices.
1. Upgrade/migrate. You could move the 8.0.4 database to the new machine and then migrate to 8.1.7. You really don't have to mess around with the control file.

2. Export/import. Recreate a database on the new machine and then import all your data. The is by far the slowest method for a large database.

Having both versions on the same server is no problem, Just install oracle in different directories and make sure your environment variables indicate the correct oracle version.