If you contact support we can usually find a way to upgrade you by paying the difference in price, but we can't guarantee that on iOS since everything we do purchasing-wise is subject to Apple's discretion - i.e. they might decide one day that we're no longer allowed to do that and then you'd be out of luck.
But if you're OK with that (relatively small) risk then yes, you can buy Basic now and contact us later if you'd like to upgrade to Professional.