There are a lot of types of subtitles
The most popular are srt and ass/ssa - they just doesn't have fps. They have timing - time when they should be shown. E.g. if you doubled video fps via interpolation timing would be the same. But if video framerate was changed just with speeding up/down (e.g. PAL's 4% SpeedUp 23.976 -> 25) timing would be different, and you have to change timing. I don't know special programs that could do such thing - but at least you can stretch them when muxing to mkv with mkvmerge.
Also there are other formats (like .sub), that are using frame numbers. You can import them to Aegisub (and choosing fps of video they are made for, likely to be 23.976 or 29.97) and export as .srt or save as .ass.
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