> It is unclear as to when the Rigveda was first written down. The oldest surviving manuscripts have been discovered in Nepal and date to c. 1040 CE.[3][78] According to Witzel, the Paippalada Samhita tradition points to written manuscripts c. 800–1000 CE.[79] The Upanishads were likely in the written form earlier, about mid-1st millennium CE (Gupta Empire period).[33][80] Attempts to write the Vedas may have been made "towards the end of the 1st millennium BCE". The early attempts may have been unsuccessful given the Smriti rules that forbade the writing down the Vedas, states Witzel.[33] The oral tradition continued as a means of transmission until modern times.[81]
As I understand it, the Tipitaka, Panini, Patanjali, etc., were also first written down around the end of the first millennium BCE or the beginning of the first millennium CE, as writing was adopted relatively late in India.
But actually there are rather solid reasons for believing that the alterations in the Rig Veda over the last 3000 years have been minimal, going far beyond what is commonly described as "pure speculation". Some of them have been described already in this thread, but there is an extensive academic literature on the topic, much of it linked from the Wikipedia article you started reading.
