Redirect all pages of one TLD to another

If you are using the R flag you are telling mod_rewrite that an external redirect is what you want, therefore the browser is asked to make a new request and the address bar should change accordingly.

Without the R flag, there is no redirect, but an Apache-internal request rewrite which is hidden from the browser. Thus, the address bar won't change. However, you cannot use internal redirects to external URIs for obvious reasons.

Since you seem to use an internal redirect anyway, just remove the R flag and it should work:

RewriteEngine On
RewriteBase /

RewriteCond %{REQUEST_FILENAME} !-f
RewriteRule (.+)$ ?url=$1 [L]

AFAIK you can't do redirection for the static assets, since GAE serves them directly according to the .yaml file rules, without even hitting your app code.

You could add a module (let's call it redirect-module for example) to your app, route ALL old domain URLs to it using a dispatcher file and use a dynamic handler in this module to redirect URLs to the new domain equivalents, along the lines suggested in the answers to the question you referenced. The new domain requests will continue to work unmodified, served either as static assets or the existing module(s) of your app. The dispatch.yaml file would look like this:

application: your-app-name
dispatch:
  - url: "your.old.domain.com/*"
    module: redirect-module

Another thought that comes to mind (I didn't actually do this, so I'm unsure if it would address your problem) is to avoid the redirect altogether and instead of mapping your app to 2 different domains map it only to the new domain and make the old domain a DNS CNAME/alias to the new domain.