I am a full-time studies graduate of Polish university. I was granted a temporary residence and work permit, am I obliged to report change in employer? Should my decision regarding temporary residence and work permit be changed if I change my employer?

You can perform work and your decision will not be changed, because it does not determine the conditions of work.

You are obliged to inform the appropriate province governor about the change in your employer in order to confirm, that the reason for granting permit still applies and that you still fulfill the requirements to for the temporary residence permit.