That's untrue. It's a geographical confusion. It would be like saying Americans' DNA was re-written or evolved in Washington, whereas we have an unbroken lineage straight back, just as the British do.
In fact, if you read Chaucer or Shakespeare, you will notice that wherever there are major differences between AmE and BrE, these writers invariably follow norms which have been retained in America but which have evolved in the UK into new forms.
It is now well established that the North and South American dialects of Spanish, Portuguese, French and English are far closer to the continental European languages of 400 years ago than their current incarnations.
Perhaps you would like some examples.
Shakespeare wrote a good deal of blank verse but if you read his rhyming verse, you will notice that it rhymes most consistently in New England English. The BBC's "Story of English" pointed this out, and professors cited there believe a small island just off Boston is the closest thing in the world to Shakespeare's actual pronunciation.
'I guess' is a phrase the British seldom use, and they regard it as very American. But you need only read Chaucer to see that Middle English speakers used it all the time.
'Gotten' is a past participle that the British seldom use. North Americans usually use this variant. Read Shakespeare, and you will see that he does as Americans today do: 'gotten' is all over his plays, just do an online search in one of the public domain texts if you would like to verify this.
Not to mention that because England has been in England for so long, you get a different accent, style, geolect and set of norms every ten miles in the country, every two miles in the city. There is no ONE British English. There are thousands.
So I find it surprising when people make assumptions about our North American English based on geographic migrations. They are incorrect.