'Decrease' seems to be a lot more simple of a word than 'reduce' as far as the definitions offered in Merriam Webster. This is in part because 'reduce' has a scientific connotation that 'decrease' does not.
From what I can tell, both verbs can be used transitively (takes an object) or intransitively (doesn't take an object). A slight difference is that 'decrease' is the same in noun form as it is in verb form whereas 'reduce' has to be replaced with 'reduction' for a noun form.
My understanding of the difference in connotation and interchangeability is that 'decrease' can't be used in all of the senses that 'reduce' can be used for. 'Reduce' has a broader use, but the two can mean the same thing in some instances.
Matt was reduced to tears.
Matt was decreased to tears. WHAT?!?!
--- I think 'decrease' can only be used literally.
Matt's paycheck was reduced by ten percent.
Matt's paycheck decreased by ten percent.
--- The difference in these examples is interesting because 'decreased' doesn't beg the question 'why was Matt's paycheck decreased?' It seems more assertive. It just decreased and that's a fact of life. Naturally, I could add "was" into the sentence to embed the same curiosity that questions the agent (who cut the paycheck and why for less?), but it's not necessary as it is with 'reduced.' I don't know how well I explained that, but that's really how confusing my brain is when it comes to work choices. :P
'Decrease' simply has a more limited range of use. I would say that the two are interchangeable in the "lessen/diminish" sense of the words, but not likely in the other senses that 'reduce' offers.
Hope that helps. :) It's more just the way I'm interpreting the definitions according to M-W.
Lastly, AP style would require that you say: cases when you would use one instead of/in place of the other one. To say 'over' implies a spatial reference (over/under), though it can technically be used to denote a preference or rank.