sql plan getting change after database upgrade from 11.2.0.4 to 12.1.0.2

we  are  encountering the issue i.e  sql plan often  getting change  after EBS database upgrade from 11.2.0.4  to 12.1.0.2  also observed at time when sql plan getting change and optimizer choosing bad plan  .

when optimizer choosing bad plan ,we are seeing at time high hard parsing (db sequential read)  which  leading to high CPU  also high swapping leading to performance issue

Please advise how we can avoid optimizer choosing bad  plan and how to avoid hard parsing and swapping