Steps to reproduce:

1. Create 2 projects
2. Create 2 field configuration schemes and assign one to each project
3. Create 2 field configurations and assign one to be the default (ie no issue type) of each field configuration scheme
4. Create a Bug issue in one project
5. Try to delete "Bug" issue type (not migrate)

You get this message:

Note: This issue type cannot be deleted - there are currently 1 matching issues with no suitable alernative issue types (only issues you have permission to see will be displayed, which may be different from the total count shown on this page).

In order for an issue type to be deleted, it needs to be associated with one workflow, field configuration and field screen scheme across all projects. If this is not the case, JIRA can not provide a list of valid replacement issue types.

It seems that this is a bug, because within each project, there is no distinction between "Bug" and any other issue type, so they should be interchangeable.