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You cant just do something so major, so fundamental to the coins security and value without first being 100% sure you are not putting the whole project at risk.
Untill 3 weeks ago ETN did not even have a block-chain specialist dev, they relied on the core team that was C++, user interface, promotion and advertising based. Nothing wrong with that when you just copy existing code, and they out sourced the security and auditing to a respected company, but that all took time, if you remember the delays on bring the coin into the world, which was in my opinion a good move.
But to now decide, to "make there own code" shows that Mr Ell`s is a great multi level marking man (as that is his company's history) advertising and promoting, herba-life, online retail resale company's, and he is good at getting his foot in the door with great sale pitches.
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You're right this is a serious matter. But I didn't read anywhere in their announcement they're going for a brand new coin. They won't create a new algo, they'll just change some parameters which is similar to editing a config file. They'll do some minor changes to reduce the "un-traceability" of transactions and increase the speed of transactions. This sure requires some testing but 1 month is normal.
Other than that, they were probably already testing these changes before the ASIC's "attack", they just happen to apply both these blockchain changes and algo update at the same time. One big update instead of two smaller is generally good for blockchains, since the the pools and exchanges who run multiple coins at the same time, won't have to do much work just for ETN's sake, reducing the chances of something going wrong.