Nodemcu ESP12-E (ESP8266), Can I reprogramming when the UART pins are defined as GPIO?
Sure, you can use the UART pins as general purpose IO pins - however, it's far from ideal - for one simple reason:


The ROM issues messages on the UART at boot time.


This means that when you power the chip up whatever is connected to the UART TX pin will receive pulses of HIGH and LOW. 

That may or may not be a problem, depending on what is connected. It's something you need to bear in mind though.