For most people (who will have the normal 9kVA supply) the increase isn't that much, the break even point rising from 5200kWh to 5710kWh - or about a 10% rise.
Basically if you use less than 5710KWh per year then there is no benefit to you in having Heures Creuse. Although this can vary depending on the percentage of electricity that you can use during the HC period. This example is based on you using a 1/3 of your usage in this cheap period and for many people even that will be difficult or inconvenient to achieve.
The bad news comes for those with a different supply. 6kVA, 12kVA and 15kVA all have huge rises in the amount of electricity you must use to break even (the worst is 6kVA where the break even point was 3743 and is now 11009 which is a huge increase). If you have this supply and use more than about 6000kVA it will pay you to switch to the more normal 9kVA because you'll pay less. In fact unless you use all your electricity in the middle of the night, there doesn't seem any benefit at all to having HC with a 6kVA supply since you'll be better off without it or switching to a more powerful supply.
Those on 12kVA supplies or more with HC should consider if they can lower their use to 9kVA (which most people can do even with electric heating), perhaps with careful management or the installation of a Délesteur to automatically switch off non critical circuits as total draw reaches 45A. The saving you'd make over a year would, I expect, more than cover your investment.
In the end I think EdF is trying to push people towards the norm which is fair enough, although it is a little sad to see lower rated supplies penalised so heavily, however just because you have 9kVA doesn't mean you have to use it (I never come close to it).