What? Why would they let you "re do" your warranty, when they tell you it's void as soon as you request an unlock code ? Did someone mentioned they should do this? (Re/un)locking the bootloader and warranty are two different things.
You can do ...
It should be possible to re-lock it like this (connect to PC in (stock) recovery mode --> fastboot oem lock begin --> flash stock ROM --> fastboot oem lock).
Also according to Motorola.
However it doesn't work on my device (XT1922-3). It returns ...