Edit: I just tested the App Store. It lets you download Sierra after having used the method below. So if you want to avoid the hassle of having to reconfigure iMessage compatibility, use this method. The only thing about it that is different than the MultiBeast/fresh setup method is purely cosmetic - the About this Mac window shows Mac Pro (Early 2008) instead of iMac (Late 2013), but System Profiler shows the correct system information (iMac14,2 etc.), and that is the information that is used for detection purposes. Also, holy crap, fast internet is fast - I finished downloading Sierra from the App Store before I even got to this sentence in this edit paragraph!
This is a cross post, but this method allows you to change over to the iMac14,2 sysdef much easier and cleaner than with MultiBeast's basic config file, which could leave you having to redo customization you did:
Just a reminder: If you're on a Hackie with a using the MacPro3,1 sysdef, you need to change to iMac14,2 (unless you have no dedicated GPU card installed, in which case, iMac14,1 will be better). Sierra won't even download on a system using the MacPro3,1 sysdef.
According to MysticalOS, the changeover is very easy. He laid it out in the following steps:
1) Back up your Serial Number from the SMBIOS pane in Clover Configurator (when reading from your personal config.plist that you created when you set up your Clover/Unibeast/MultiBeast install). A text clipping works wonders here. Save it in a safe place for future reference so you never have to deal with iMessage/App Store authorization issues down the line.
2) Back up your MLB number in the Rt Variables pane. Same deal as in Step #1.
3) Switch to the SMBIOS pane and click the wand on the right side.
4) In the window that pops up, click the picture of the iMac and select iMac14,2 as your system definition. Do not change any fields. Click OK when done here.
5) In the SMBIOS pane, copy your backed up serial number and paste it into the appropriate text field and hit Enter to force it to register. If you fail to do this, your machine will look like a new machine to Apple and you'll have to go through iMessage fixing headaches as well as authorizing a new computer, which may be difficult if you're very close to your five device limit with your Apple ID account.
6) Check the Rt Variables pane and make sure your MLB is the same as it was. You can verify it against the text clipping backup you just made in Step #2. If it changed at all, just copy the text clipping you made earlier and paste it into the MLB text field and hit Enter to register it.
7) Save the config.plist by pressing CMD-S or using the File -> Save menu item.
7) Reboot. You now have an iMac14,2 using your MacPro3,1 serial number.
This is the "easiest" change in that it lets you keep your setup as-is without having to go through the deauthorization/authorize new computer headache when switching to a new system. Your "About this Mac" will show MacPro3,1 as it goes by serial number (as does the App Store for purposes of authentication for downloads), but everything else that requires system identification will show iMac14,2, which you can verify is fully changed over in System Profiler.
Note: Check your Memory speed in System Profiler after rebooting to the new iMac14,2 configuration. On my system it disabled XMP detection (Clover was set to No when using the MacPro3,1 sysdef yet it still natively worked), which would reduce your memory bandwidth considerably. If you are not seeing your memory's "rated" speed, but instead 1300/1333 MHz, open up Clover Configurator with your EFI partition mounted and your config.plist selected and switch to the Boot pane. From there, click the Use XMP Detection button and press CMD-S to save. Reboot, and you should have your memory bandwidth back.
If you go through this carefully, the entire process for changing and checking your new iMac14,2 sysdef should take no more than five minutes for a slower person, two minutes for a very fast person that knows their way around Clover.
On the plus side, it's been reported that the iMac14,2 has full power management for Haswell CPUs vs. partial stepping with the MacPro3,1 sysdef, so you may enjoy better energy savings as well. You might also get fully working sleep too if you didn't have it before.
Oh, and if you play Blizzard games you will want Sierra on day one for improvements to Metal.
