Did this modification tonight - it took all of 5 minutes with the right screwdriver to hand and was very simple. Definately a DIY process - no need to send to Canon.
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I modified my 3 TS-E lenses myself, and it went fine, until the last screw on the last lens. The last screw was very tight, and as I applied a little force downwards, to get the screw to move, the tilt mechanism moved (despite being locked tightly), moving the screwdriver off-axis, and resulting in the screw stripping. I then had to send the lens away to have the stripped screw replaced (no big hassle, cost £20 + P&P).
Therefore my tip is: before attempting to loosen a screw for the first time, lock the tilt mechanism not at the central position (no tilt), but at the maximum tilt, such that any force you apply cannot make the tilt move further. You might not need it, but I sure wish I'd thought of it beforehand.