[KJKingJ]

I often use subtitles (often in ASS/SSA format) on content i'm streaming from Plex and have no issues changing the subtitle format mid-stream - if the subtitles are being burnt in because the player (e.g. Plex Web or mobile apps) can't natively render them then there's a few seconds delay before the video resumes. On players that can natively render them (e.g. Plex Media Player or Kodi with the PlexKodiConnect plugin), the changeover is instant.

[jacobush]

For me while playing, there is only one subtitle to choose from. The one I chose before starting the movie. No matter how many subtitles the file actually has. I'm using the Apple TV Plex app.

[zbuttram]

From experience, that delay when you try to switch while it's using burn-in subtitles can increase quite a bit on slower servers, but as you said the switch does work (eventually).