Moving a corner OF'er to CF opens up a huge hole at corner OF, and while finding a top corner OF'er through free agency or trade is usually easier than finding a SS, both would be costly.
What about filling the position from within the system? This is also a consideration in making an either or choice.
Bogey's contract ends after 2019.
#6 prospect: Marco Hernandez (still pre-arb)
#7 prospect: CJ Chatham
JBJ's (and Betts') ends after 2020.
#36 prospect Steve Selsky
#28 prospect Aneury Tavarez
#29 prospect Tate Matheny
#30 prospect Kyri Washington
#31 prospect Tyler Hill
#32 prospect Lorenzo Cedrola
I realize a lot can happen over the next 2-3 years, in terms of prospect growth and acquisitions as well as trade acquisitions, but as of right now, I wouldn't feel horrible about having Marco & Chatham as our SSs of the future, assuming the money saved helped us keep one or two of Sale, Betts, JBJ or Porcello.