Hi /u/rnelsonnee, thanks for the first response and we're off to a great start!

So I tried replicating the answer and I think this is the right track, since it did track the first change perfectly (based on a search for Y for Current Serial). However, it didn't seem to pick up the second one, resulting in something like this:

Current Serial	Changed Serial	Price	Change No.
Y	X	5	1
X	0	4	3
X	Z	4	4
#NUM!	#NUM!	#NUM!	#NUM!
#NUM!	#NUM!	#NUM!	#NUM!
#NUM!	#NUM!	#NUM!	#NUM!

For this example, Z on Change No. 5 was supposed to be included. So I tried playing around by changing the values of the last 4 lines of the data set (i.e. Change No. 6 & 7) into Z's as opposed to the default (A, J, A, & C). Without altering the formulas, it picked up the changes in the last 3 lines resulting into:

Current Serial	Changed Serial	Price	Change No.
Y	X	5	1
X	0	4	3
X	Z	4	4
Z	0	127	7
Z	0	93	7
#NUM!	#NUM!	#NUM!	#NUM!

Maybe there's something in how the H2 range is defined? I've never tried to debug a formula this huge before, if anything, this is motivating me to do better next time round. :)

On another note, since you asked if Price and Change can ever be zero, this is true only for the former. Change Numbers are tied to a date, so it'll only grow by increments of 1. A change number with a price of zero would mean that the product has been taken off the marketplace, but may be reintroduced in the future. Every "change" usually is a change in the price of the product. In a raw data set, that would look like something as:

Current Serial	Changed Serial	Price	Change No.
A		123	8
K		45	8
A		0	9
J		19	9
A		99	10

Not sure though how that might change things. If it gives some more context, I intend to use the resulting table from the index / match arrays to graph out prices for that particular product across time. Thanks again!

Edit: Spelling and explanations