It does not work that way in R. i believe you need to set up a separatee install of Rstudio with the old R in a separate project.
You can however choose a version of R in R studio in options and run your project, but if you need to run multiple projects with others in new versions it will confuse R and things will get wobbly in more than one project.
Safer to just run it together and probably I would choose a separate installation in a different partition, whatever it takes
Note: I am just a regular R user, and I do not develop for R.