So I'm transferring from one HDD to another HDD.

Yes there will be no difficulties Since you are using your same gamertag. I did this without any difficulties upgradeing from 120GB to 320GB. I cannot believe than I filled up a 120GB HDD so I upgrade to a larger HDD. good luck to you.

Yeah that's fine, you only need to transfer your licenses when you change your console.

http://support.xbox.com/en-US/xbox-live/marketplace-and-purchasing/download-content

I found this just a bit easier. I got a slimmy last Summer and put the 120GB awkwardly into the console's floppy slot. You can also re-download the stuff. When you do this you also make all this stuff playable, for the most part, by other Gamertags on your console.