The Soft Altitude function on the A320 is only +/- 50ft (DSC 22-30-70-70 p2-4).
My first guess would be since the ALT CRZ was already captured, changing the MCDU to a new cruise ALT is not affecting the current mode. To enable a return to ALT only, I suspect you could set VS+0, then type the new CRZ ALT in the MCDU, then reselect the current level with the FCU. It should get back to capture the current altitude with ALT only. Be careful though as to verify the actual ALT blue on FMA!
