You can't create a new project that re-uses a previously used project key in the same Jira instance. So for example, if your project key was originally TEST and you renamed the key to DEMO, you can't then create a new project with the TEST project key. Jira actively won't let you do this. This limitation is explained in the Editing a project key documentation. So before you get worried about the attachments, you would still need to overcome this project key limitation in some way first.
And since Jira has an expectation that an imported project has to use the same project key that was in use at the time the XML backup was created, I can't see a way for you to import this data without potentially overwriting your current project data there.
Perhaps you could try a different approach here. Instead of adjusting the destination Jira site, have you considered changing the project key in the source instance first, and then creating a backup there. This way you could then create a brand new project in the destination instance with this new project key value and import your data there. At the time of the project import, you would need to check on the path of the attachments to confirm that this works, but I think it could help here.
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