I would really appreciate your advice on this matter.

I have a macbook pro early 2007. Its old i know and last year i replaced the HDD with a 420GB SSD as the HDD failed. It was a fairly straightforward install.

However, physically it is falling to bits and the screen casing is broken. Still runs fine though.

SO my generous girfriend has bought me a new Macbook air pro 13 inch 18ghz, 8gb, 128gb laptop. Initially i was delighted about this but i have something of a quandary.

The HD is only 128gb in the macbook air. No where near enough the space i need on a computer. My music collection alone is this. 

The bottom line is, can I take out the 420GB SSD drive in my old mac running El Capitan and simply install it in the new Macbook air?

In theory it sounds easy, replace the hard disk, startup the new Air computer and then update the OS to the latest one as the old mac only goes up to capitan.

Any advice greatly appreciated.

Many thanks