Until recently, upgradation
was not an standard (acceptable) term. From a dictionary
1.(computing, nonstandard, proscribed, chiefly India) Upgrade; upgrading.
Enhancements and Upgrades depend on the author of the document/spec.
In my experience, things are enhanced by changing or adding functionality. Especially in vendor provided/supported products. When an organization decides to move from an older version/release of a product, it is upgraded to the newer version/release provided by the vendor.
Within an organization, enhancements are usually done in a test/development environment. When this is ready to promote, the Productoin environment is upgraded.