One of them is the use of a close central vowel, [ʉː] or thereabouts rather than a fully back [uː], in words of the GOOSE set.
Long before this recent development, Daniel Jones pointed out, getting on for a century ago, that English [uː] is not as back as the cardinal-type quality that one hears in German. (Compare German du, or even French doux, with English do.) In my 1971 PhD thesis I commented on the backness of Jamaican [uː] as compared to what is usual in England. But it was Caroline Henton who first properly documented the new fronting of RP GOOSE, in her 1983 article ‘Changes in the vowels of Received Pronunciation’, JPhon 11:353-371.
Where does this development come from? As with most other sound changes, nobody really knows. But Jones pointed out in his Outline, nearly a century ago, that an ‘advanced’ variety of uː was used after j (see scan).
So perhaps what happened was essentially a takeover by this allophone, which became the default realization in all positions and thus displaced the backer variety used in words like spoon and food in conservative speech.
Since then things have gone further: today one can indeed hear unrounded mid or front varieties of GOOSE.
I’m fine. How are ˈjɨː?