I have a bit of a unique situation. I lived with an ex and during our time of living together, my original 360 red-ringed and went to that place in the sky that all 360's eventually go. Because of this, she bought a shiny new 360 Slim to replace it.

I had an upgraded 60gb drive on my old 360 so I used a transfer cable and dumped all my profile/save data on to that shiny new Slim and then sold off my 360 stuff.

Skip to a few months later, she leaves me and I'm left without a 360. My plan however, is to buy a new(ish) one soon.

All of my save data and everything is on the 360 Slim but I'll have a new 360 most likely before I see her (wasn't a clean break) and will begin playing some new games, Forza 4 and GoW3 of course.

If I start a new profile on my new 360 and eventually get all my save data from the old console, will it be a clean merge when I dump that data onto my shiny new console?

tl;dr Need to take my data from a 360 that isn't mine, move it to a 360 that is mine, preserve newer data on my 360 during the process.