Well, APEX has functionality to export pages and individual components. So for one example, you've been good and kept the schema name, workspace names consistent. You also maintained the same application ID amongst environments.
So, you do an page export from one environment, and try to import it to the other, but you receive the following error:
This page was exported from a different application or from an application in different workspace. Page cannot be installed in this application.
These components were exported from a different application or from an application in a different workspace. The components cannot be installed in this application.
For a page or component export respectively.
The missing link?
You need to have consistent workspace ID's amongst workspaces!
Once you have consistent workspace ID's amongst environments, when importing components, you will find you have better success!
Great, now you know what to do in the future. What if you haven't done this for existing systems? Don't despair, you still have options, albeit, a bit more lengthy!
You need to export the application that has all the new enhancements in it, and import it into the same workspace where the target application is, but don't replace the existing application - instead just auto assign a new ID. Now, in this case, instead of doing page exports and imports, you need to use the copy function that comes with apex.
So go to your target application and in page designer go to the Create toolbar and select Page as Copy.
Then specify Page in another application
From here, you will specify the application you had just imported and then the page that contains all the enhancements you are after. Follow the steps and you will find the page from the other application in your target application.
Not as elegant as if you had consistent workspace ID's in your environments, so from here on in.. consistent workspace ID's ;-)