View gcloud configuration:
$ gcloud config list
You should see something like:
[core]
account = your.account@gmail.com
disable_usage_reporting = True
project = myproject-somename-123467
Your active configuration is: [default]
Unset the current cloud project
$ gcloud config unset project
Set the new project. Your current App Engine name is something like: my-other-project.appspot.com
Just use the first part without the appspot.com
$ gcloud config set project my-other-project
Run gcloud app deploy now
$ gcloud app deploy
You should see it now say something like
Services to deploy:
descriptor: [/Users/yourname/Source/Repos/appengine/app.yaml]
source: [/Users/yourname/Source/Repos/appengine]
target project: [my-other-project]
target service: [default]
target version: [20181218t202435]
target url: [https://my-other-project.appspot.com]
Do you want to continue (Y/n)? y