Nodemcu ESP12-E (ESP8266), Can I reprogramming when the UART pins are defined as GPIO? I wonder if there may be any problems using the pins that the UART are using as GPIOs, if now I want to be able to reprogram the circuit?
(Of course I will detach the ESP12-E from the mainboard before so)
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.