1. A method for upgrading application packages installed to an application platform and managed by an on-demand database service, comprising: responsive to receiving from a first subscriber a selection of an application package that includes metadata components, installing the application package to the first subscriber on an application platform of the on-demand database service; receiving, from a developer, a new version of the installed application package, the new version of the installed application package including (i) any new metadata component or components relative to a prior version of the new version of the installed application package and (ii) any changes to a metadata component of the prior version of the new version of the installed application package; enforcing restrictions on modification of software entities in the new version of the installed application package, wherein the restrictions prevent the developer from making invalid changes to custom field types and application programming interface (API) names of the installed application package; informing the first subscriber of the new version of the installed application package; and responsive to receiving from the first subscriber a selection of the new version of the installed application package, installing the new version of the installed application package on the application platform.