I'm cloning new altcoin. Everything goes nice.
I already have a working wallet and even premine some coins.
But I stumbled in one problem.

Let's say I want to have 125 million coins at all. 100 million is premine and 25 million for staking.
So on start, I premine 100 million using regular PoW. And then I have to turn off PoW and left only PoS?
Because if I left PoW, than everybody will be able to mine new coins easily like me on start. I can't change mining difficulty later because difficulty is built in genesis block.
And if I set big difficulty from the start that my premining will take a years.

So how should it work?