I copied slash marks and pasted them, assuming that the chord changes would be attached, as in Sibelius. They aren’t.
I tried filtering but don’t understand it.
Thanks for your patience; my third day using Dorico.
You can draw a marquee selection around chords, or use the System Track (plus the little square at the end of it) to select everything within a region, then Filter. You can also select one chord symbol and the use Select More.
It’s intentionally difficult to select chord symbols; when the functionality was first introduced it was all too easy to inadvertently paste some chord symbols in the wrong place, overwriting the chord symbols in all instruments. The big difference is that (with occasional exceptions), Dorico treats chord symbols as a global track, whereas Sibelius treats chord symbols as glorified staff text.