(Corrects spelling of first name to Erik (not Eric) in paragraph 21): LONDON (Reuters) - Bitcoin’s value has plunged by three-quarters this year, sending the original and biggest cryptocurrency back to levels not seen before its bubble. And price isn’t the only aspect of trading that has changed. The retail investors behind bitcoin’s dizzying ascent to a record of nearly $20,000 last December have fled, leaving the early adopters and crypto-related firms that traditionally dominated digital coin trading driving exchange volumes.