N Cook wrote:

http://home.graffiti.net/diverse:graffiti.net/clip.jpg

Looks like when the replacement transistor isn't yet soldered
to the board you can lower it enough onto the board to lever
open the spring and shift the transistor upwards under it.
Messy procedure, though, with heat conducting paste applied.

I would seriously consider to mark the hole of the transistor onto the
heat sink, take that off the board, drill a decent hole and to secure
the transistor onto the heat sink with a screw and washer like it was
always done in valuable devices :-).

Regards,
H.