Been there. Done that. Bad experiences. So what if they notice that the seal was broken! I would be doing them and us a favor.
They could even just shut down your account, after you refuse to pay full retail for the HWS you attempted to replace with a new HDD, for violation of the agreement.
This is Charlie-ville: It can be a great place with sunshine and love, but if you violate terms, especially with regards to the HDD (which can be construed as attempting to illegally copy copyright content), then, YES, Dish may very well drop the hammer, and I would NOT blame them as they would be protecting their relationships with content services and owners regarding removal of HDD's with copyright content.
Your attempt at what seems reasonable to you, the replacement of an HDD, could be seen by Dish as "over the line" in their view. Oh, and as stated by another poster, Dish would likely charge you the full retail for the HWS even if you cancel your subscription as the door hits you on the way out (the door hitting you is Charlie's view, not mine), and the CC company is likely to DENY your request to not remove the full retail HWS charge after they investigate.
Please, save yourself agrivation and just call Dish and have them send a new HWS.