i have just found this in FCOM4
4.02.30 P7
Descent Speed Profile:
if the pilot revert tp SELECTED speed/MACH mode during descent, the profile is NOT modified and the aircraft flies the same profile at the FCU selected speed/mach value.
that means...it is better to drop below the profile couple of hundreds of feet to cater for the extra knots when starting the desceleration.
if not...if you like to continue on the DES mode...speed brakes might be required for the speed reduction.