Oracle Database Upgrades and Migrations Metalink  ID 1152016.1 

Oracle Upgrade note

Note 1561791.1 Upgrade / Downgrade Assistant: Oracle Database/Client
Note 419550.1   Different Upgrade Methods For Upgrading Your Database
Note 551141.1   Database Server Upgrade/Downgrade Compatibility Matrix
Note 730365.1   Oracle Database Upgrade Path Reference List
Note 455744.1   Best Practices to Minimize Downtime During Upgrade
Note 739485.1   How to estimate the time required to upgrade a database ?
Note 1515747.1 Oracle Database 12c Release 1 (12.1) Upgrade New Features
Note 1493645.1 Oracle Database 12c Release 1 (12.1) DBUA : Understanding New Changes With All New 12.1 DBUA 
Note 757245.1   Can you Upgrade RDBMS and Convert From 32-bit to 64-bit Binaries Directly on Linux or Windows based Intel Platforms Using the Database Upgrade Assistant (DBUA)
Note 1320966.1 Things to Consider Before Upgrading to 11.2.0.2 to Avoid Poor Performance or Wrong Results
Note 1392633.1 Things to Consider Before Upgrading to 11.2.0.3 to Avoid Poor Performance or Wrong Results
Note 785351.1   Oracle 11gR2 Upgrade Companion
Note 601807.1   Oracle 11gR1 Upgrade Companion
Note 466181.1   10g Upgrade Companion