Hi Mandar,
I tried to reproduce the behavior in house on SDM 14.1 + CP04 environment. I created an incident ticket with Priority one value and a ensured that an priority SLA is attached to the ticket. I waited till the first Event triggered and moved to complete status.
Then changed the priority value to 2 and noted that the second set of priority SLA services are attached to the same ticket. After refreshing the ticket noted that the priority 1 related SLA service types are all cancelled / completed and the priority 2 related SLA service types are still in progress.
Probably you may want to try this on a plain vanilla system for the correct behavior in your environment. Its working as per the OTB behavior in house.