Context
EK application has a module that store personal documents for user. When user account is deleted, those documents may be transferred to another account.
To achieve that, we need to alter the user account cancel form when building the form, validating and submitting it.
Let's review the 3 steps.
BUILD
The form before altering it looks like this
We need to add a field to select another user account to which the document of the canceled account will be moved to.
To achieve that we Implements hook_form_alter() in MyModule.module: