Oh, wait, don't bother. I was able to update my older Mac to Sierra. I'll test subtitling on it tomorrow.
I wasn't able to update it previously because the Bootcamp partition, where I ran Avidemux on Windows to get the subtitling, was preventing OS update. But since the subtitle filter's been fixed for mac, I deleted the Bootcamp partition, got a whole lot of hard drive back for my use, and updated to Sierra.