Hi,
It is allowed to change the support condition even for concrete frame element.
However, the designer need to ensure the changes are reasonable and justifiable.
The designer need to ensure the oversll structutal stabiliy will not be affected.
For structural where there is not core wall...
1) when the user want to change the support condition of frame element (column or beam).
2) Support condition depend on the designer design intention. There are some cases where support condition for column/beam need to be pin instead of fix. Example such as steel column connect to RC column...
