I currently have a 6 pre-set headunit with the aux input only - I'd like to swap it out for the standard 9 pre-set unit with bluetooth as I'm not a huge fan of using aftermarket aux bluetooth thingies.
Not impossible, but completely impractical.
You're better off finding a suitable aftermarket HU that does all you need.
The changes from the low-line stereo to mid-line stereo are substantial, as none of the very complex wiring is there, not even the fuse box has the tappings for the extra units needed for the conversion. It's not impossible to, but the gain you get aren't worth the extra effort, when you can go aftermarket with much less work.
The Bluetooth on the mid-line is for phone calls only, you can't stream music, so you're not gain much except better sound reproduction from 9 speakers rather than your current 6 speakers, functionality remains similar.
The mid-line stereo uses different main door speakers, different tweeters, a subwoofer in the boot, a different HU with a 6 disc MP3 capable CD drive, 2 radio receivers with diversity function, a bluetooth phone module and a 300 Watt amplifier to drive the speakers, plus all the wiring to connect everything together including a MOST ring for audio data.
The low-line HU has just that, a HU with integrated 4 X 25W max outputs to some nasty cheap door speakers. You can have steering wheel controls if not already fitted, but that's about the limit of upgrades for it.
Helpfully having the low-line HU does mean it's much easier to upgrade to a better unit, as the wiring isn't so complicated, and you can always fit an amplifier and buy the factory sub to improve the low frequency response over the standard door speakers, which again are easily replaced with better Bowers and Wilkins drivers from the Jaguar XF.
