Moving existing resources to an Amazon S3 bucket

I'm having trouble trying to serve all images from an Amazon S3 bucket ('assets-cdn') in Runway 3; I've copied all images from my local resources folder to an Amazon S3 bucket and set this as the default bucket in config.php - these were initially uploaded to the default bucket prior to setting up the S3 bucket. I've also set the PERCH_RESPATH to the S3 cloudfront web path, i.e.

define('PERCH_RESPATH', 'https://cdn.hiddenireland.com');
define('PERCH_DEFAULT_BUCKET','assets-cdn');    

All appears fine until I want to edit an existing image: when I edit a blog post in Runway, the existing image loads up fine and is being served from the Cloudfront CDN web path as expected. However, when I select and save a new image (from the default bucket because nothing appears under the new 'assets-cdn' bucket), the image is missing from the post as all the versions of the image are uploading to the local resources folder as opposed to the 'assets-cdn' S3 bucket.

Only if I choose an image that was uploaded to the 'assets-cdn' bucket, do versions of the image get saved to the correct location, i.e. to the S3 bucket. So to make all images in the assets panel to appear under the 'assets-cdn' bucket so that they are saved in the right location, can I just update resourceBucket column in the perch2_resources? i.e. run the following query on the db:

UPDATE perch2_resources SET resourceBucket='assets-cdn' WHERE resourceBucket='default'

Does this look safe enough? Also, do I need to add the buckets="assets-cdn" attribute to all the image tags in my templates or does setting the default bucket to 'assets-cdn' in config.php cover this?

Big thanks! A

Summary information

Perch Runway: 3.0.10, PHP: 5.6.31, MySQL: mysqlnd 5.0.11-dev - 20120503 - $Id: 76b08b24596e12d4553bd41fc93cccd5bac2fe7a $, with PDO
Server OS: Darwin, apache2handler
Installed apps: content (3.0.10), assets (3.0.10), categories (3.0.10), perch_blog (5.6.1), perch_forms (1.9)
App runtimes: <?php $apps_list = [ 'perch_blog', 'perch_forms', ];
PERCH_LOGINPATH: /perch
PERCH_PATH: /Users/adam/Sites/hiddenireland.com/site/perch
PERCH_CORE: /Users/adam/Sites/hiddenireland.com/site/perch/core
PERCH_RESFILEPATH: /Users/adam/Sites/hiddenireland.com/site/perch/resources
Image manipulation: GD
PHP limits: Max upload 32M, Max POST 32M, Memory: 128M, Total max file upload: 32M
F1: 3b606135b33e6a102526838f4152a807
Resource folder writeable: Yes
HTTP_HOST: hiddenireland.dev
DOCUMENT_ROOT: /Users/adam/Sites/hiddenireland.com/site
REQUEST_URI: /perch/core/settings/diagnostics/
SCRIPT_NAME: /perch/core/settings/diagnostics/index.php