I am currently running integrated graphics with an AMD A4 7300 APU. I am looking to upgrade my system with an AMD Athlon X4 860K CPU and a Sapphire R9 270. Do I need to reinstall elementary OS after the hardware upgrade for hardware recognition or will the new hardware be recognized?
The only main issue : your video graphic card. If you want to change your graphic card, do not forget to uninstall your driver.
No, not usually. I have switched hard disks between different machines more than once, and that shouldn’t be a problem. The only thing to make sure beforehand might be to see whether your new hardware needs drivers that are definitely not included in the distribution you are going to use.