If I understand the problem, he wants to replace a part in a project that is in the temp bin with a replacement part that is in mine bin with different pin assignments and not lose the traces... or something like that.
You are right, the .fzp would need to be identical in both parts in order to route the traces as assigned in the .fz which in the .fzz. If the pin assignments in both .fzp files are the same and a little magic they can be swapped out... other wise you would need to use the Delete Minus.
There are a couple of ways to swamp out parts... but first you need to understand how it works. When a project is saved as an .fzz, a .fz file is created, this is your project and how it is wired up. If you have parts that are not core parts, the .fzp and the four .svg files for that part will be added to the .fzz. So if you had two non-core parts in you project you would have a total of eleven files in the .fzz. The .fzz is nothing but a .zip with an .fzz extension. When the project is opened up in Fz the non-core parts will be saved in the partfactory and appear in the temp bin whether you have the original part in your mine parts or not. If the original part is in mine parts it will use that one, if not it will use the one it the partfactory. The funny thing is, that they seem to be connected, although they are two identical parts with the same variant#, but they are not really connected. When the project is closed the temp parts in the partfactory will be deleted. When you re-save the project, it will create a path to the part in mine.
I found a little bug while looking into this project; If you click on the temp part and click add to bin, you can add it to My Part and it will create a link in your my_parts.fzb where the part is located "partfactory". But when you close your project the part is deleted from partfactory. The next time you open Fz and click on My parts you will get a error stating the part does not exist. Then you have to manually delete the link from your my_parts.fzb.
To get around this problem you will need to export the part form the temp bin and import it back in the My parts and all the part files will be installed in your parts/user directory.
To swamp part out, the two parts ( the old one and new one) must have the same family name but with a different variant#. Even a temp part with the same family name will show up under the same family group. Just click on the new variant and the new part should snap right in place regardless of which bin it is in...
Remember, in Fz you generally need to close Fz and re-open it to clear out the buffers or update your changes...