You can only change the Parent OKR to one that is in the same OKR time period
So if an OKR is Q1 2019, it can have a parent OKR that is either
- in Q1 2019
- or is annual OKR set to 2019
It cannot be set to a parent in any other time period
Other reasons are
- Company OKR can only have another Company OKR as a parent
- OKR is in an previous period and the OKR Grace Period is over so cannot be edited