I can't believe that Yard would replace Nowell. He's had a great season for the Chiefs and for England, where as Yard hasn't done much of anything anywhere, his last great contribution being 2 years ago in New Zealand. I would really feel for Nowell if it is the case.
As for the talk of Nowell playing as a 13, that's a pipe dream. He's been plying wing all season for the Chiefs and that's where he will be for England, and it's not going to change. Full Back may crop up, but I can't really see it. If anyone moved it would be Watson as he plays there for Bath more often then not.
Back when Nowell and Watson were in the U20s together (2013) Nowell was always 15 while Watson was on the wing. I get that Watson's been playing more fullback than Nowell in recent times, but I think before 2019 we can expect Nowell to move to 15 for Chiefs in place of Dollman. I'd also argue that Watson is a far more natural wing, as he's faster, while Nowell is better defensively, has a better kicking game and I'd say is better under the high ball. A no brainer as to who I'd shift to fullback when Brown's phased out.
People are also saying that Nowell isn't fast enough to be an international back three, but I think that's unfair. Le Roux and Kearney (In his prime) have both been excellent in international rugby and neither of them are particularly pacey. AAC too, never been too physically impressive, yet he has 100+ caps for Aus. I can see Nowell being England's AAC, able to cover 13, wing or fullback and performing exceptionally wherever you put him. Nowell's only 23, he still has lots of time to improve and also to move to 15. I see many England caps and many great performances from him in an England shirt in the future.
Edit: Looking through England U20s teams over the last few things is revealing some interesting details. Clifford always used to start at 8 over Vunipola for the U20s. Daly was selected at 13 over JJ, JJ was shifted to the wing to make way for him, or just left out all together. Nowell 15 over Watson who stayed wing (Already mentioned that though). Hill always at 12 over Devoto, unlike the Saxons ATM. Chris Cook used to get in at 9 over Robson. Kvesic was rated higher than Cane in the 2011 final, yet Kvesic has 3 caps for England, Cane has captained the All Blacks. Short and Wade on the wings over Yarde. Vunipola used to play tighthead, with MArler loose and Jamie George hooker. May was a 13, older Watson and Wade wings and Homer 15 (Hella fast back four).
Sorry if that's mostly spam, I found some of it quite interesting and thought you all might want to have a read.