Set Warranty Bit: Kernel So whenever I boot up my device or restart it I see a little yellow message in the top left corner of the screen that reads "Set Warranty Bit: Kernel." I have researched this issue and learned that I must reflash the firmware of the phone.

That being said. If I reflash the correct firmware for the same model for the same carrier, will my phone still have service following the procedure? Or will I have to work with my carrier to reactivate my phone?
Short answer: you don't need to worry.

Provided you pay your bills, your carrier does not care much, if at all, that you root/ flash Roms and such. No extra steps will have to be taken either; its all on the sim, and flashing ignores that. Keep note you have lost your warranty, though!