If I am concerned about minimizing downtime I would use AP image pre- download option.

>> I will download 7.6 image on old WLC and push this image to the APs connected to them. When Pre download is completed APs will have primary and secondary image as 7.0 and 7.6 . At this time both my APs and WLC are working with 7.0 image.

>> Swap the image on APs.

>> Check if we have right image on APs( show ap image all).

>> Now in downtime reboot the controller. APs will right away join with 7.6 image(this will save time what APs take to search for new WLC, download the code and then reboot and join the WLC).

>> Once the APs have joined the old WLC on 7.6 then we can move the APs to new WLC using HA tab.

Link : http://www.my80211.com/cisco-wlc-cli-commands/2011/2/20/wlc-predownload-the-image-to-the-access-points-from-the-cont.html

I hope that helps.

Thanks,

Ishant

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Old WLC = WLC1

New WLC = WLC2

Here's a proven process: 

1.  Configure WLC2:  Make sure WLC2 has the same firmware as WLC1.  Copy the config of WLC1 to a notepad.  Change the IP addresses.  Copy the new config to WLC2. 

2.  Upgrade the firmware of WLC2 (including the FUS/bootstrap).  

3.  Make sure you configure DNS for WLC2.  

4.  Connect WLC2 to the network and make sure you can remotely access WLC2.  

5.  Upgrade the firmware of WLC1 but DO NOT reboot. 

6.  Pre-download the AP images.  

7.  Move the APs from WLC1 to WLC2. 

8.  Once the APs have moved to WLC2, reboot WLC1.