Moving from on premise to the cloud or vice versa may require architectural changes and programming, so an upgrade is the best time to make these changes since you will be doing some re-architecting of your solution anyway.
You can choose from four deployment models:
1. On premise -- traditional deployment in your data center
2. Dynamics CRM Online -- in the Microsoft cloud
3. Run Dynamics CRM on Microsoft Azure cloud infrastructure
3. Third party hosting -- hosted by another provider
The deployment model also affects your product licensing strategy. Microsoft offers flexibility which may help with moving from one deployment model to another. For instance, Dynamics CRM Online subscriptions may be used to cover on-premise licensing while you migrate from on premise to cloud deployment. The InfoStrat Dynamics CRM cost calculator provides licensing and subscription costs for on premise and CRM Online.
If you are upgrading from Dynamics CRM 2011 (or 4.0), you will likely be rewriting code that is no longer supported in 2015, allowing you to optimize performance for a cloud deployment. Microsoft provides insights on this topic in What's New for Developers.
Cloud deployment offers different approaches to integration than on premise. You also have different ways to tune performance in cloud deployment than conventional on premise deployment.
Tackling deployment options along with your upgrade is a great way to save time and money in the long run.