I queue a unit for training, I do not instantly train it that round. I pay 100 crystal to put it in the queue.
Before the unit is ready, I research improved versions of gear that the unit can use.
The unit gets removed from the build queue. I do not get a refund on my crystal costs. I cannot put it back in the queue because I need 150 crystal to put it in the queue and I only have 20 crystal.
(numbers are approximate)
I believe that this mechanism contradicts the expressed purpose of upgradeable gear (convenience).