I _would_ call moving from a TI-83 to the TI nSpire CAS an upgrade
Having used all of TI-83, TI-84, and TI nSpire, I can say that unless you need and are allowed to use the CAS portion of the nSpire, it is a downgrade in almost every way.
In calculus, I could be done with a calculation on the TI-83 before my classmates in on the nSpire even got to the relevant screen - the UI makes molasses look fast. It may have more features, but at least at high school levels, you just straight up don't need them.
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Having used all of TI-83, TI-84, and TI nSpire, I can say that unless you need and are allowed to use the CAS portion of the nSpire, it is a downgrade in almost every way.
In calculus, I could be done with a calculation on the TI-83 before my classmates in on the nSpire even got to the relevant screen - the UI makes molasses look fast. It may have more features, but at least at high school levels, you just straight up don't need them.