Explanation
As the instance_type is hard-coded in source module, you will not be able to change its value from destination module. Instead of hard-coding you should use variable with default values.

Terraform module need not be public and open-source. Module can be placed in -
* Local paths
* Terraform Registry
* GitHub
* Bitbucket
* Generic Git, Mercurial repositories
* HTTP URLs
* S3 buckets
* GCS buckets
https://www.terraform.io/docs/modules/sources.html

resource "aws_instance" "web" {
ami = data.aws_ami.ubuntu.id

Explanation
The terraform taint command manually marks a Terraform-managed resource as tainted, forcing it to be destroyed and recreated on the next apply.
Reference: https://www.terraform.io/docs/cli/commands/taint.html

workspaces allow conveniently switching between multiple instances of a single configuration within its single backend. They are convenient in a number of situations, but cannot solve all problems.
A common use for multiple workspaces is to create a parallel, distinct copy of a set of infrastructure in order to test a set of changes before modifying the main production infrastructure. For example, a developer working on a complex set of infrastructure changes might create a new temporary workspace in order to freely experiment with changes without affecting the default workspace.
Non-default workspaces are often related to feature branches in version control. The default workspace might correspond to the "master" or "trunk" branch, which describes the intended state of production infrastructure. When a feature branch is created to develop a change, the developer of that feature might create a corresponding workspace and deploy into it a temporary "copy" of the main infrastructure so that changes can be tested without affecting the production infrastructure. Once the change is merged and deployed to the default workspace, the test infrastructure can be destroyed and the temporary workspace deleted.
https://www.terraform.io/docs/state/workspaces.html
https://www.terraform.io/docs/state/workspaces.html#when-to-use-multiple-workspaces