changing star constraints
 

is it illegal to modify charted altitude constraints displayed on MCDU in order to achieve a relevant T/D and thereafter a constant angle descent to the runway?
(these constraints typically disappear when you are cleared to a point down route on a STAR)

In the USA it is normal to get cleared"descent via STAR". If that happens you are required to comply with all speeds and altitudes on the chart (These same constraints should be in your MCDU.). The only change you could (but probably should not) legally make is changing window/at-or-below/above to hard altitudes.