Actually you can do this. You dont have to upgrade the old server.
If you upgrade a database on a server you have to do the following steps:
1) install the new software on the server
2) prepare the new environment for the upgrade (e.g. spfile)
3) prepare the old database for the upgrade
4) shutdown the old database
5) startup the database in the new environment (startup uprade)
6) apply the upgrade procedure to the new database
If restore it a new server for upgrade you have the following steps:
1) install the new software on the NEW server
2) prepare the new environment for the upgrade (e.g. spfile)
3) prepare the old database for the upgrade
4) RESTORE the database to the new server to the point in time you want (but after the point in time where the preparation for the upgrade was finishe) but don' t open it
5) OPEN the database in the new environment (alter database open upgrade;)
6) apply the upgrade procedure to the new database
So RMAN helps you to transport the database to the new server but it does not help you to avoid the upgrade steps. You can leave your old database open but all changes that are made after the point in time you recovered to are left in the old database and therefore lost.
There are some documents from Oracle Support that cover this topic
- Frequently Asked Questions about Restoring Or Duplicating Between Different Versions And Platforms (Doc ID 369644.1)
- Complete Checklist for Manual Upgrades to 11gR2 (Doc ID 837570.1)
- Restoring a database to a higher patchset (Doc ID 558408.1)