Both 上がる and 挙がる are read the exact same way — as あがる — and in the dictionaries I looked them up, they appear under the same entry, and seem to show up as alternate spellings of the same word.
Is is the case that either spelling works on all occasions (meaning they're completely interchangeable), or are the spellings used in different contexts/circumstances?
This question does mention some variations, but since it is a rather broad one, wanted to get some more clarity around the specific nuance between the two writings.