It's not a question of it
can't be done as it
won't be done -- as the contests in the west coast primarily have been done successfully with absentee or mail-in ballots.
The states postponing elections aren't extending the time period of absentee voting in the meanwhile -- they're simply holding back the date to June or something and holding an ordinary, presumably just as contagious, in-person vote then.
The DNC has little control over how states manage the mechanics of the primary election itself, but this comment
In some states, it is literally impossible without amending the state constitution.
strikes me as dubious.