[ECS] [request]: Ability to modify the load balancing configuration for the existing ECS service

### Community Note

* Please vote on this issue by adding a 👍 [reaction](https://blog.github.com/2016-03-10-add-reactions-to-pull-requests-issues-and-comments/) to the original issue to help the community and maintainers prioritize this request
* Please do not leave "+1" or "me too" comments, they generate extra noise for issue followers and do not help prioritize the request
* If you are interested in working on this issue or have submitted a pull request, please leave a comment

**Tell us about your request**

The ability to modify the load balancing configuration for the existing ECS service.
Currently, ECS doesn't support doing that by management console and aws-cli; it notices the limitation message `Load balancing settings can only be set on service creation.`.

**Which service(s) is this request for?**

ECS

**Tell us about the problem you're trying to solve. What are you trying to do, and why is it hard?**

For now, we have to recreate a new service entirely if it needs to modify the load balancing configuration. I deem it'd be nicer if we could just upgrade them, instead of remaking.

**Are you currently working around this issue?**

No, at present, we have the only way: "recreating".

The request concerns changing load-balancing configuration on an existing ECS service through the management console and AWS CLI. No repository files, tests, or implementation entry points are identified, so first confirm whether ECS supports this operation and where the roadmap tracks service-level changes. Done would mean updating the configuration without recreating the service.

Written by the indexing model from the issue text.