So apparently Mercedes found a loophole in the rules and decided to change power units on Hamilton's car in-between every practice session. This gives him a 55 place grid penalty for this race, but it also gives him 3 brand new power units for the rest of year. Clever way to avoid future grid penalties, but I imagine a rule clarification will be forthcoming.
The FIA also confirmed a new rule that will stop teams from introducing several new engines on the same GP weekend in order to have them available for the rest of the season, as Mercedes did with Lewis Hamilton in the Belgian Grand Prix.
"Only the last element fitted may be used at subsequent events without further penalty," said the FIA. "This is to prevent the stockpiling of spare power unit elements."