The migration process and doc you are referring to is old, it should still work but there are ways to get it done with out doing it that way.
With the new SZ code it might be easier, depending on your AP to controller topology. If all your APs are connected to a network where you have control over the DHCP server or local DNS server you can use DHCP option 43 to redirect the ZD AP to the vSCG and once in SCG mode, you will need a secondary Option43 (see KB article on how to set that up) to redirect the APs (which should have been converted and defaulted) again to the same vSCG IP.
When you bring up the new vSCG, make sure you enable LWAPP2SCG conversion and to have the process default the AP. This is one of the question that is asked during the cluster setup. I also recommend using a 1 interface vSCG-H for configuration simplicity, but any other flavor should work.
If possible I would do this:
ZD IP: 192.168.1.100
vSZ-H (vSCG-H): 192.168.1.101
At AP network:
DHCP Option 43 sub option 3 > 192.168.1.101
DHCP Option 43 sub option 6 > 192.168.1.101
1. Sping up the vSZ-H, enable LWAPP2SCG conversion with the default AP as an option
2. At the AP locaiton setup the DHCP option 43
3. On the current ZD, set the Access Point Policy to limit the discover to the primary ZD and backup ZD.
4. put the Current ZD IP as the primary ZD, put the new vSZ-H IP as the secondary "ZD"
5. After 5-10 min, disconnect/shutdown the Primary ZD, let the AP connect to the Secondary "ZD", which should trigger the LWAPP AP to SCG AP conversion, which should default the AP.
6. The newly defaulted AP will look the vSZ-H base on the Option 43 sub option 6 and now should be able to join the vSZ-H.
The above steps will allow you to migtrate your ZD network to SCG (vSZ-(H or E)/SZ100/SCG200), You will then need to confgigure the vSZ's Zone/wlan/AP and move the AP from the staging zone to the final zone. In the case of vSZ-E/SZ100, the AP will br put in the default AP group using the default wlan group.
I hope this hleps.