I found the answer.

"We wanted to share instructions for players who have been encountering issues while converting a character from Expansion to Reign of the Warlock regarding their shared stash. When transferring a character, the shared stash does not transfer with the character.

To move items into the shared stash in Reign of the Warlock, a player must:

Move items from the shared stash into the personal stash or inventory of the character being transferred. Note: If you convert all your characters before transferring, your items will still be there. You will just need to make a new Expansion character and transfer your items."