finally after doing test i was successfull to change DB Owner for Mirror DB.

My DB Mirror is setup for High Performance(No Witness)

For each DB, did following steps.

1> Backup DB on Principal server

2> On principal server, change High Performance to High Saftey-- cLick OK

3> On Principal, Manual Failover to Mirror Server

4> sp_changedbowner on Mirror Server for that DB---will encounter error msg 10054--- run

sp_changedbowner again & that will change\fix.

5> After Changing owner, failover back to Principal

6> On Principal, Change High Saftey to High Performance-- cLick OK

Was able to do the same for all remaining DB.

I hope this will help others for there future reference.