Each version of Nuxeo comes with bug fixes, new features and improvements.
This means upgrading to the latest public release is a smart move.
In order to make upgrade easy we are very careful not to break anything.
Remember that we provide support and use Nuxeo EP in a lot of project, so if we break something, we have to fix it :)
So basically:
- we don't break the APIs between two versions: we add new APIs and deprecate old ones,
- there are several minor versions between the deprecation and the removal so you have time to adapt your code,
- if we completely replace a service (that was the case of SearchService and EventService for example), we provide compatibility packages so you can continue using the old API (even if migrating to the new API is highly recommended).
In terms of Data Migration we are also very careful not to break anything.
Once again, we run Nuxeo EP for a lot of our internal needs and we upgrade them very frequently and don't want to have data migration issue.
Anyway, when some changes or optimizations impact the storage layer we either:
- make the upgrade automatically,
- or provide guidelines for the upgrade.
Upgrading is usually a simple and painless process.
Using the template system also allows to easily transpose your configuration from one version to an other.
In the worse cases, in case of problem, Nuxeo Support is there to help you 
For upgrade steps, check the KB Upgrades section where you will find upgrade procedures per Nuxeo version.