Successful HD replacement, iMac late 2013
Internal hard drive failed
Went very well. Took a while to break loose the display adhesive, but perseverance prevailed. I agree with the commenter who said no need to remove the speaker to replace the hard drive. Only had to move it aside a bit. I replaced the failed hard drive with a SSD. Only tedious, not really difficult, part was breaking loose the adhesive and removing the old adhesive. The YouTube video on installing the new adhesive strips was very helpful. The iMac is better than new now with the much faster SSD.
Use the iFixit tools. Test before installing the new adhesive. Watch the YouTube video on adhesive strip placement.