In 'unless', how would've 'negative connotation' caused 'on' to change to 'un'?

Solution 1:

This is almost a form of rebracketing-- in which a word's roots are re-defined by misunderstanding, like alcoholic (which has roots in al-kohl and the suffix -ic, but was rebracketed as alco-holic, allowing for the suffix -holic to appear in words like workaholic or coffeeholic)-- crossed with a mondegreen-- a mishearing leading to respelling, like eggcorn for acorn, or like in the game Telephone. As the prefix on- became relatively obsolete and the phonetically-similar prefix un- gained popularity, the on- in onless got redefined as un-, incorrectly. This is an example of a false etymology.