You get to use the term upgrade when you are going a level up on the same release. Say for example that you want to move your database from 8.1.6 to 8.1.7. You would call it to be an up grade since you are still on 8i release i.e 8.1.x.
On the other hand you would use the term migrate when you move from one major release to the another. i.e say 7 -> 8 (8.0.x) --> 8i (8.1.x) --> 9i (9.0.x)
When you say that you did an upgrade, as I mentioned above, you would be moving up within the same release. This would not bring in any major changes, intead it would be some patch application or some minor new entries. When you say migrate, it would mean a major feature changes,