After more digging I found these instructions by ShaneEEV:
1) Insert USB Stick into front panel USB socket
2) Put SDG into System / Info / Upgrade
3) Select telnet file for SDG2042X and UPDATE
4) LEAVE POWER ON SDG
5) On remote computer console, type: telnet <SDG IP address> <SPACE> 10101 <RETURN>
6) On remote computer console, type: mount -o remount,rw ubi2_0 /usr/bin/siglent/firmdata0
7) On remote computer console, type: cp /usr/bin/siglent/firmdata0/NSP_system_info.xml /usr/bin/siglent/firmdata0/NSP_system_info.xml.orig
 On remote computer console, type: vi /usr/bin/siglent/firmdata0/NSP_system_info.xml
9) On remote computer console, move down to the line that starts with <license> and press 'i' for INSERT and right arrow over to just after </license> and press the delete button until the line only has </system_information>
10) On remoter computer console, press the ESC key, then type::wq <RETURN>
11) On remote computer console, type: sync
12) on SDG: remove USB stick
13) on SDG: turn power OFF
Item #9 above should be:
The line you need to change looks something like this:
<license><bandwidth_update_license>XXXXXXXXXXXX</bandwidth_update_license></license></system_information>
You should change it to look like this:
<license><iq_support_update_license>TRUE</iq_support_update_license></license></system_information>
This does update the frequency limit to 500 Mhz, and the unit identifies as SDG5052X.
BUT, IQ modulation is still NOT enabled. How do I generate the license key for that?
Many Thanks,
luudee