Moving Yelich to the role of McCutchen seems wise, assuming they don't acquire a bat. McCutchen played 50 games in the OF, Yelich could play a bit more. He shouldn't play 154 games or get 671 PA like he did this year (McCutchen played 134 games with 580 PA). He should sit against some lefties.
Moving Yelich to 1B doesn't seem great. 1) There's no guarantee he'll play 1B better than Rowdy (who would get moved to DH); 2) He's not a platoon match for Rowdy; 3) He won't have a good enough bat to fit at 1B long-term.
Long-term, Yelich is a 4-5th OF and DH. Unless his bat somehow is reborn (BTW, if he were traded to the Cardinals, that would be sure to happen).