Video/Audio merging question

[pjpjpjpj]

I have an old copy of a movie that was a DVD rip. It has surround sound but is only DVD quality video.

I downloaded (from PlayOn) the same movie in 1080p, but PlayOn downloads are only stereo.

Is there any freeware or cheap-and-easy software I can use where I could replace the stereo audio track in the HD file with the surround sound track in the old DVD file? Obviously I'd have to work on syncing and whatnot, but I first want to know if I can even do it.

And would I have to extract the surround audio first (into a multi-track audio file)? Or is there software that can take two movie files and let you choose the video from one and audio from another and merge them?

I've googled around a bit and the only thing I saw that looked like it could do it easily is Adobe Premiere Pro, but I have to imagine there are others, hopefully freeware.