Hi @Daniel_Goldner,
Welcome to the RStudio Community Forum.
When you knit foo.Rmd to Word format, if the Rmarkdown code completes successfully, the file foo.docx should be opened and displayed immediately in Word. There is no need to do a separate download and/or rename - the Word file already exists in the working (or project) directory. If this isn't happening for you, then something out-of-the-ordindary is going on. Have you changed any settings? Is Word installed on the machine?
Could you post the offending foo.Rmd for testing?
***Edit: just noticed you are using RStudio Cloud - not sure how that changes things.
HTH