Done this a couple of months ago, as you say, you deffo need the new surround. To remove the older square type you need the four removal keys(more to that later) once you've slipped the headunit out you need to disconnect the multiplug. Bit of a pig 'till you look at it once it's out; looking at the unit put your right hand behind at the lower edge and find the bootom of the multiplug, there is a sort of lever lock, pull this away from the unit and up and it starts to free the plug, wiggle it the rest of the way out. Two torx head screws undo the old style surround.
Part one........done!
As you look at the new unit you will see it mounts by four screws to the dash/HU housing. This is where the problems begin, you can't mount it like that. The physical structures are completely different,I purchased the mounting clips for the newer unit thinking they may help, no chance!
So here's the bodge.
As said the old surround is fixed by screw, the new one is a tight friction/clip fixing. I took the unit out to the car to power up to check the code and make sure it worked. Sat it in the hole and it looked quite snug, so I thought, hmm, presented up the new surround and gave it a dunt.
Sorted!
Yes it's sitting in on it's own weight and secured by the tight surround. Been in for months now, driven hard over poor surfaces to no avail, still solid.
The keys, these are a tenner from team torqesteer, but if you wanna give me your address i'll send you mine to use and you can send them back when you're done.
HTH
Kenny