Using InStr To Find Number Within A Number?

I work for a store and we're migrating to a new Point of Sale system. The new POS uses the entire UPC code while our existing one uses only partial UPCs, so I have to update from the partial to the full. ex. Partial 3378093 - Full 0033780935

The partial UPC will always be included in the full UPC, the difference is the dropped leading zeros (and possibly other numbers) and the check digit number at the end. What I have is a spreadsheet with with two worksheets (Old, New) - one has our old UPCs and other information, the other has the new, full UPCs and more information we'll need. What I want to do is write a function that will search through the full list of new UPCs to find the one with the old UPC inside it and return the full UPC. InStr seems the right function to do this - correct?

The part I'm having trouble with is writing something that will search through the full column of new UPCs using InStr. I've searched through the forum for a similar situation but the few I've found aren't working or appropriate. I don't want to include any code I've written because 1) it doesn't work and 2) it would be way too embarassing!