I doubt a great deal will change.
The primary result of Myriokephalon was to seriously dent the hitherto irrepressible self-confidence of Manuel I, and to discourage him from making further serious campaigns: but given that by the late 1170s the Emperor was an old man by medieval standards, this would certainly have happened at some point anyway. Even if there's no Myriokephalon, Manuel's most unlikely to be campaigning in the field for much longer.
There'll be little butterflies, of course, and little butterflies can flap big wings. If you have a butterfly that gives Manuel another three or four years of life then you're obviously going to change a lot, but a victory at Myriokephalon is not going to guarantee this.