Yeah, I decided on HSOC during my second year. The major is pretty small, and they don't require any certain GPA or anything (to my knowledge) to be admitted into it. If they do, it's probably very low.
Given its small size, you get a lot of attention from professors and administrators (advisors, etc). You can really shape the major however you want to, and you largely have free reign over what classes you want to take.
I don't see why you wouldn't be able to complete everything if you transfer in your junior year, but you may want to email one of the HSOC administrators to make sure. Dr. Greene or Dr. Barnes would be your best bet (their emails are on the HSOC webpage).
Also, the Capstone requirement really only takes one semester to complete, so it's not a huge part of the program. You can fulfill it by simply taking an upper level class and doing a research project for that class, approved the course prof.