How to conditionally rename spell or magic item?

[AncientOne]

Suppose I want to

1. Test to see if a specific thing (e.g., spell or magic item) is defined.
2. If it is defined, rename it.

For example, if the Apparatus of the Crab is defined (because the user has purchased Ultimate Equipment), then I want it to appear as Apparatus of Kwalish. However, if the user has not purchased Ultimate Equipment, then I don't want the user to get an error when my user-content is loaded.

How would I go about doing that?

I know I could make a copy of it and have the copy replace the original, but that would leave me in the position of redistributing someone else's work, which seems... less than entirely moral, especially if the content I'm copying is for-pay rather than community-provided.

Thanks!