I've just done exactly what you're wanting to do
@Hendry I got a couple of the Mk7 key fobs from eBay and a couple of the genuine GTI logo metal parts.
The blade from the mk5 needs filing down slightly to fit into the rotating part on the mk7 as it's 0.2mm too thick. You can of course get the mk7 blade cut at a key cutters to match your original key. I did this after first grinding and filing the mk5 blade but wasn't happy with the fit. If you take your time though you should be fine.
All the internals from the mk5 fit into the mk7 fob perfectly.
Once you've got the mk5 key open you need to cut off the melted over tabs that hold the circuit board in place. I found that a section of blade from a Stanley knife/exacto blade, (dunno what you English folk call them) is perfect. You want the snap off blade type and then just use one section snapped off.
To get the transponder out you have to prise off the vw logo to reveal a screw which you undo to open the top half of the fob.
Then to get the transponder out: it is nestled in a slot which is then melted over to capture it. I wasn't sure the Stanley blade method would work and I didn't want to break the glass vile that it's in. So I used a blade type end on my soldering iron to push down and melt away the sides of the slot which it sits in. I guess you could also heat up the tip of a small flat head screwdriver and use that. Then the glass vile can easily be pried out.
Disassembly of the top part of the mk7 fob is basically the same. You'll see the slot for the glass vile. Once in just melt the edges over a bit to hold it in place.
The rest is all straight forward.
If you're getting the keys from eBay I found a seller that sells them with a vw logo sticker as well that goes over the screw holding the top part together. Most don't seem to come with that.
