"I had to do this because the file I was using was getting too slow as the game was getting longer"
This is transparently untrue. You can upload more than one file on this game page. You promised the full game for one low payment, now you're asking for twice as much.
If you wanna do a sequel then finish up this plot and then release a sequel. Don't just decide to double the price and lock out all existing customers unless they agree to pay twice.
We're not talking a lot of money here (although many people will have paid over the advertised price), this is about saying your provide something, then deciding you weren't going to fulfill your promise unless people pay again. Why would I pay for more when you didn't fulfill what you said you would the last time I paid you?