For example, I created a LAMP stack with multiple EC2 instances using Cloud Formation. Now, I just want to assign a temporary Security Group to one of the instances for next day or two. Can I do this via AWS Console? Will this cause any issues with future Cloud Formation update?
You can use the AWS Management Console to update pretty much any aspect of an AWS CloudFormation stack, see Updating a Stack. While the most common approach is to have things that are anticipated to change modeled as input parameters, it is also possible to modify the stack template itself in principle:
If you want to modify resources and properties that are declared in a stack template, you must modify the stack's template. To ensure that you update only the resources that you intend to update, use the template for the existing stack as a starting point and then make your updates to that template. [...]
However, as hinted upon already, not every change is allowed, and the effect varies:
If your template includes an unsupported change, AWS CloudFormation returns a message saying that the change is not permitted. This message might occur asynchronously, however, because resources are created and updated by AWS CloudFormation in a non-deterministic order by default.
Each property of every AWS Resource Type specifically states whether or not a resource can be changed and whether an update requires No Interruption, Some Interruption or a Replacement (see section Updates to stack resources in AWS CloudFormation Stacks Updates for details).
Quick answer yes.
This type supports updates. For more information about updating stacks, see AWS CloudFormation Stacks Updates.
It is not totally clear in the documentation if you can do this without interruption however I know if you were to add the ingress rule to the security group by hand in the AWS web console (a test I have performed in the past, but not recommended as it causes divergence between your cloud formation script and the actual running code) it will cause you zero interruption.
Yes, you can use the AWS console to modify anything that was created by CloudFormation. You can also use different CloudFormation templates to launch various AWS resources. Here's how: enter link description here