Also, rename will change the original name of the file to your new name.
Save As makes a copy of the original file and names it the new name.
If the part is in an assembly and the assembly is loaded in memory when you do either operation, the assembly updates to the new name. The same holds true if the part is shown on a drawing and the drawing is loaded.
If you are running TeamCenter or another PDM system, the options may be a little different than running NX in native mode.
The worst thing you can do is do the Rename or Save As in the OS.
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