Morris DID NOT play a change of pace role last year. He had 99 carries when Zeke was suspended, and only 16 carries the rest of the year. He played the main RB role while Zeke was suspended. He did this when the Dallas O-Line was notably not playing like the Dallas O-line due to the Smith injury. He averaged 4.8 YPC.
Breida DID play a change of pace role (behind Hyde) behind the SF Oline, which finished the year top-10 in run blocking per footballoutsiders.com. During those last 5 games when "Breida started to come on," he averaged 4.5 YPC, and had 3 catches (TOTAL, not per game) for 44 yards.
So, lets agree that Breida was the COP back in 2017. As a COP back, he had 19 carries (18% of his 2017 rushes) in "passing" situations (1st & 15+ yards, 2nd or 3rd and 10+ yards). On those 19 plays (when the D's primary focus probably wasn't the run), his YPC was 4.9YPC. With regards to receiving, Breida was already top-20 in YPR for RBs in 2017 (min 20 catches), while his catch rate was only 58%.
I would think we can also agree that when a player increases his workload, his efficiency numbers usually go down, not up. So, since Breida got a boost from getting carries as a COP back in passing situations, and already had a relatively high YPR (with an extremely low catch rate), it makes little sense for either his YPC or his YPR to improve if he is going to get MORE carries, presumably in more traditional running situations, with the Defense keying on the run.
There's not a lot of logic behind your projections, other than "it's his second year." His YPC was inflated by running against D's focusing on the pass. His YPR was inflated by a small sample size where 1-2 long receptions can drastically alter the efficiency rate. It's more reasonable to assume a decline in efficiency with more work, rather than a boost.