From the March 24 Update Notes:
Top tier SDU upgrades can now be purchased and will no longer disappear after ending a session.
On one of my characters, I had previously purchased all the bank and backpack SDU upgrades, and had 39 backpack slots available (which were lost when the session ended). Now, my character only has 36 backpack slots available, and can’t purchased the last SDU to get the 39 slots either. Can’t confirm the bank side as I don’t know what the counts should be.
Haven’t tested yet to see if I can purchase up to 39 on my other characters and keep them yet.