Yes, if you make changes to a custom page activity, you need to rebuild your app and upload it to the market as a new version. (The Android market will take care of getting that new version of the app out to your users, though.)
If you want to be able to change the content of an HTML page remotely (without rebuilding your app), you need to host your HTML at a web server somewhere and show it using the Website activity.
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If you want to be able to change the content of an HTML page remotely (without rebuilding your app), you need to host your HTML at a web server somewhere and show it using the Website activity.