Unfortunately, unless something has been changed recently, if you launched with the setting of recordings saved to the internal microSD card, without a microSD card inserted, you cannot switch to "record to internal memory" instead in flight, without first landing, and changing the location of recordings setting to internal memory. It does not auto select, or default back to internal memory without a microSD card inside. You must select "use internal memory storage" before flight, if you realize you left home without a microSD card inside, and that 8GB will quickly be used up!
It also will not roll over, and use the 8GB internal memory, if you fill up the microSD card during recordings while in flight.
There should also be a red warning icon on the screen, showing a microSD card with a line through it, if you have no microSD card inside before launch.
Gone, and lesson and functionality learned the hard way.
Sorry for your loss.