Can a separately excited dc motor changed into self excited (series or shunt) ?

 

If the stator is intended for the same voltage as the rotor then it will work just fine as shunt wound.
It is unlikely to work properly as a series wound motor as the stator will be designed for lower currents than the rotor.

 

How would you intend to self excite a wound field?
If you rely on residual magnetism you can get a runaway condition, possibly to destruction.
How would you produce a self exited series motor?
Max.

 

Are you trying to build something that will power itself?