There hardly is a straightforward answer to your question. It all depends what customizations you've applied to the built-in modules and how your custom ones were implemented. There are resources that could help you with a sophisticated answer to your question. Please refer to the Sitefinity Upgrade Center
for a whitepaper and several videos on the upgrade subject.
Grisha 'Greg' Karanikolov
Do you want to have your say in the Sitefinity development roadmap? Do you want to know when a feature you requested is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items