I just bought a new 15" MacBook Pro, brought it home, followed the Migration Assistant instructions on my old 13" Retina MacBook Pro running Mavericks (which explicitly say to install all software updates on the source machine before migration; I did so, including an OS X update), powered on the new computer, and when prompted to do so connected the two via Ethernet (using two Thunderbolt adapters; I do not have a Thunderbolt-to-Thunderbolt cable).
The old Mac appeared on the new Mac's migration screen, but with a warning sign over its icon. When I clicked it, I got:
This source contains a newer release of OS X. Your Mac requires an upgrade before you can migrate from this source.
I've found sources saying this can happen when you upgrade to Yosemite, but I have not done so. I'm just running Mavericks, up to the newest software update. So I assume the issue is that the old Mac is running a newer Software Update.
How can I proceed with Mac-to-Mac migration? I would like to avoid having to purchase new equipment if possible; is there some way to update the new Mac even though I haven't set it up yet?