Hello Madhu
You can first start with Technical upgrade where move all your Transports from MDG 7 to MDG 8. You may find various issues related to UI but you will get OSS note for most of your issues.
Once the Technical upgrade is over then test all the scenarios in MDG 8 with the same old MDG 7 roles. Once the system is ready then you can plan for functional upgrade.
Its always better to use hub approach as it has lot of advantages. You can go through the link suggested by Henk.
Kiran