svn relocate error -- invalid relocation destination

According to TortoiseSVN the relocate command shouldn't be used if:

You want to switch to a different branch or directory within the same repository. To do that you should use TortoiseSVN → Switch.... Read the section called “To Checkout or to Switch...” for more information.

Therefore, you should perform svn switch https://my-repo-server/new-proj-name/branches/my-branch since you are moving within the same repository.

Solved it with a stopgap solution -- from within the root of the working copy, deleted the .svn folder and ran svn checkout <new-url-of-branch> .

If anyone has a better answer that doesn't involve deleting the .svn folder, I will select it.