<speculation><p>1) cells share a large portion of DNA sequences<p>2) cells change their behavior based on their environment
(Mark Ptashne "A Genetic Switch: Phage Lambda Revisited")
(Mark Ptashne and Alexander Gann "Genes & Signals)<p>3) The technology exists to edit the genome
(Ali Movahedi "One Introduction of Genome Editing (CRISPR))<p>4) modify yeast genome so that based on the sugar concentration
they produce barley rather than yeast<p>5) combine regular yeast and "barley yeast" in a sugar concentration<p>... beer!<p></speculation>
This is like wondering if somebody could use a text editor to turn the "cat" binary into a Postgres server - this isn't "modifying" the genome, it's replacing it outright.