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Been there, done that, would not recommend
(feddit.org)
Its definitely a bad idea writing new code that builds up on your old code, that has not been tested properly, because you quickly have to start debugging multiple layer is code at once.
This! Also, it's a solid base to stand on when you want to optimize your code. Performing optimizations becomes much easier, quicker and fraught with errors once you have a unit test suite that will instantly tell you whether your change is working or not.