RWU is spot on.
Your descent path is still assuming 280 knots. Flying selected 320 knots at this stage will have you flying the DES profile with a bit of thrust.
If you have been cleared to an intermediate level and level off for a bit, you could do the following to optimise:
-maintain selected speed
-force the aircraft into the cruise phase again by inserting the present level on the PROG page (or just enter '1')
-modify the speeds on the DES page to 'current mach'/320
The descent profile is recalculated now and you could do everything managed.
Now the reverse scenario:
Managed descent speed 280kts, ATC reduces you to 250 or 220 a long way out, back to selected speed initially. Descent profile still assumes 280kt. Aircraft in descent phase.
-make a vertical revision at any descent waypoint and replace the standard 250/10000 by e.g. 220/'present level or higher'
Now your profile is recalculated and assumes the 220 kts. You could do everyting managed again.
However you will always have selected speed and OPEN DES/VS at your disposal! I usually just use the 3 times table and some allowances for wind etc. and forget about the FMGS profile.
S.