Here's my first rudimentary working code that selects the changebar Pgf (just one at this point), copies it, and pastes it into a second changes document. Thanks for your help Jang.
// findChangebar() returns Pgf object that has a changebar.
changePgf = findChangebar(doc);
if (changePgf) {
var tBegin = new TextLoc();
var tEnd = new TextLoc();
var chgRange = new TextRange();
tBegin.obj = changePgf;
tBegin.offset = 0;
tEnd.obj = changePgf;
// This constant gets you the end of the paragraph.
tEnd.offset = Constants.FV_OBJ_END_OFFSET;
chgRange.beg = tBegin;
chgRange.end = tEnd;
doc.TextSelection = chgRange;
doc.Copy();
// Text selection now held in clipboard, available to other docs.
// changesDoc is already open.
var firstChgPgf = changesDoc.MainFlowInDoc.FirstTextFrameInFlow.FirstPgf;
var changesTbeg = new TextLoc();
var changesTend = new TextLoc;
var chgRange = new TextRange();
changesTbeg.obj = firstChgPgf;
changesTbeg.offset = 0;
changesTend.obj = firstChgPgf;
// I found it was not necessary to select to the end of the paragraph, so offset 0.
changesTend.offset = 0;
chgRange.beg = changesTbeg;
chgRange.end = changesTend;
changesDoc.TextSelection = chgRange;
changesDoc.Paste();
}