this does work. it only seems like it doesn't because you have to manually refresh the equipment page in order to see the result.
To do this either exit and reenter equips, equip and unequip the necklace, or switch tabs and back.
If the necklace didn't change it's because the upgrade failed, but you will still see that your gems are being consumed meaning that it does work and it is faster than waiting for the animation to finish.
I've gotten 16 +9 necklaces this way in less than 120000 gems.