Making more of Migration Assistant
Can I use Migration Assistant to transfer my data from my old MacBook Air (that's running Mavericks) to my newer MacBook Air (which came with Yosemite installed) Or must I upgrade my old Mac to Yosemite first (I would rather not update to Yosemite on the old one just yet.)
Well then, today's your lucky day! Migration Assistant in both Mavericks and Yosemite can copy data between your new Mac and another Mac running a version of the Mac OS as old as Snow Leopard (OS X 10.6.8). Apple suggests that if you want to migrate data from a Mac that's running a version of OS X created before Snow Leopard, you either update that Mac to a compatible version of OS X or transfer the data manually.
While we're on the subject, there are a few additional things you should know.
Same name: If you're transferring data between two user accounts that have the same name ("bob" and "bob," for example), you can choose to replace the "bob" account on the new Mac with the "bob" account from the old one. Or, you could set up a completely new account in addition to the existing "bob" account--one called "robert," for instance.
You'd choose the first option if the "bob" account on the new Mac has no files on it that aren't on your old Mac. (They'll be overwritten, if so.) And, as you might guess, if you've already put a load of stuff in the new Mac's "bob" account and want to keep it there, it makes sense to create an additional account. You can then copy files from one account to the other, if you like.
Making connections: If you have a lot of data on the old computer, it has a Thunderbolt port, and you have better things to do than to wait hours for your data to move from one machine to another, I urge you to get a Thunderbolt cable and use it to connect the two Macs. If you'd prefer to save the 29 bucks on the cable and don't mind your Macs working away as you sleep, feel free to use a Wi-Fi connection. (But if you have a lot of data it still may not copy from one Mac to the other in the space of one night.)
Or use Time Machine: If you've been backing up your old MacBook Air to Time Machine you may be able to skip this whole computer-to-computer rigmarole. Migration Assistant is happy to pull data from a Time Machine backup as well. (You'll see that option when you run Migration Assistant on your new Mac.) Ideally, you'll have your Time Machine drive plugged directly into your new MacBook rather than access it over the network as speed, again, is an issue with a network transfer.