[Ninja_En_Short]

Don't worry dude, the TAR-21 being hard to upgrade is a legend, no more no less.

I fixed one back in France it's not hard to modify. It is basically a V3 with the motor on the rear rather than on the bottom.
Getting the gearbox out may be hard the first time because the gun is a little more complex then usual but don't worry : it is not a pain in the ass like the FAMAS.

Just carefuly put each screw on a carboard to know were they came from and you'll be done in no time.

EDIT :

In fact you don't have to open the gearbox at all : it's ARES so it has the quick spring changing system. Use a screw driver to get the spring guide and spring out (push and turn 90°), put the front of the new spring in, place the spring guide at the end of it, push hard to get everything in and turn 90° to lock the spring guide.
DONE !

Look in the stock where the battery goes or around there, you might see the gearbox and the spring guide.

I had that on my former G36... and man I love these gearboxes !