Whenever I try to use the replace component command, everything seems to work ok, but after saving and closing it will ALWAYS revert back to the previous model/assembly. Any thoughts or Fixes?

For Example, if I replace a ball valve with a needle valve in an assembly, it will go through the mate references, and then after I click ok, it will replace the valve fine (this is in all assemblies, not just routing). However when I save the assembly, and close it, it will ALWAYS default back to whatever the previous part was when I first made and saved the assembly.