To what degree can the Rare Earth Elements be interchanged in its workaday applications? In their humdrum pedestrian "high-tech applications due to their unique magnetic properties", how interchangeable are the Rare-Earth Elements (REEs) if, e.g., the US lacks one of them? 

RRautamaa comments on Pentagon in talks with Australia on rare earths plant.


  Rare earths are not rare, but they're chemically very similar. (This is because when you go to higher atomic numbers, they start filling the $\mathrm{f}$-shell, but the $\mathrm{f}$-shell is lower in energy than the $\mathrm{d}$-shell, so it's "hidden" by the $\mathrm{d}$-shell. The $\mathrm{d}$-shell is the valence shell and is the same in all of them, so their chemistry is almost the same.)
  
  Separating two chemically similar substances is expensive. Separating 15 of them is much worse. This means that the mining part is easy, but the processing plant on the site costs about $500 million. This is why rare earth mines are so few in number.


This question was spurred by the Pentagon's halt on May 22 2020 on rare earths funding program pending 'further research'. "Of course, the problem with rare earth elements has never been scarcity. They're actually pretty common. The problem is just that the cost of mining them exceeded their market value everywhere but China, so China ended up with a monopoly and the ability to drive up the price. So it's not clear that this changes much.
It depends on the particular use of the "rare earth". Mischmetal, containing an assortment of mixed lanthanides, can be used as-is for lighter "flints" or fire-starters. On the other hand, ultra-high purity lanthanide elements are needed to get specific spectral lines for lasers, for example.

Other uses, such as in magnets and in high-temperature superconductors, may be somewhat less demanding of purity. Didymium glass, for example, used in glasswork to block yellow sodium light, is a mix containing primarily samarium, praseodymium and neodymium.

Another expense involved in lanthanide production is separation and disposal of the radioactive isotopes from the ore. At one time, for example, gas lantern mantles contained mildly radioactive $\ce{ThO2}$, but those are no longer available in many countries due to concern about the alpha emitting thorium and the alpha and beta emissions from daughters.