If your project decides to go commercial the initial license term will include a cost of conversion based on the quantity of code already written.
It would be interesting if they expressed that rate as something like:
A gz -1 compressed tarball of the affected source code will be created, the size (in bytes) multiplied by X is the maximum fee we would charge for retroactive licensing.
I choose the wording 'maximum' there as it still allows them to offer better deals under other circumstances.