Different series of the system have different issues. This series of system is one of the harder units to open. Here's the IFIXIT guide: iMac Intel 27" EMC 2546 Hard Drive Replacement
Here's another little writeup on HD upgrading: "Tapered Edge" Aluminum iMac Q&A - Updated July 27, 2014.
As far as the replacement HD with the exact same drive: Apple uses custom firmware so one can't just drop in what appears to be the same HD. While there are newer HD's now available which are compatible you will need to make sure it's one that is compatible.
Your other option here is getting a converter adapter. Here you can use anyones HD. OWC makes such a device OWC In-line Digital Thermal Sensor for iMac. While their guides show this is for the '11 model it in fact will work for all of the newer models. The only issue here is making sure the new HD does not push your systems temp higher than your older HD (review the HD spec sheets),
Since you have a fusion setup you can't disconnect the HD without effecting the SSD. You will need to break the fusion setup first. I strongly recommend you make a complete backup of your data before you do anything (otherwise you could loose something). I also recommend you setup an external USB thumb drive as a bootable drive with the OS installer. You may want to get a Thunderbolt or USB external case, or instead a SATA to USB adapter to connect your new HD so you can prep it up first before opening your system.