If the map is the exact same map just in a different set you can go to the diff
> Right click the difficulty > edit > Top left corner File > Open .osu in notepad > (If you want all the timing points and the hit objects then copy from the Timing Points to the end of the HitObjects).If you want just the Hit Objects then just select all the hit object numbers and copy.
> Go to the new mapset > edit the difficulty > file > open .osu in notepad > paste the contents (either over the existing contents or just the pieces you are wanting to change).
Hopefully this is what you were asking for. If the map is ranked/pending and is not made by you then make sure to not hit "update beatmap" as it will update to the websites version of the map (whatever that might be)