Ah, such a simple question, such a complicated answer!

First off, search for the thread on the 5k/5k/15k promo; this issue is discussed there. (I assume this is why you're asking, right?)

Let's see if I can put this briefly:

On the old website, refaring generated a new ticketing date. I know this because (a) the date changes on the e-receipt, and (b) M+ denied me a double RDM bonus once because they claimed that my "reticketing date" was the important one, not the original date of purchase.

On the new website, however, it's murkier. The e-receipts say the ticketing date is the original date of purchase, but further down, it has the date(s) when change(s) is/are made, so perhaps the repricing date is the new ticketing date.

There is a trick, though, to print out an e-receipt via the new website that shows the refare date as the ticketing date: Request an upgrade using 500-mile certs for any part of the trip (you can always go back and cancel if you don't want to use them). On the next screen, where it shows your request as pending, there's a "View E-receipt" button; if you click this, out pops a receipt in the old style, which shows the ticketing date as the refare date. If M+ gives you trouble later on, you've got paper proof.