While copy/paste in the datagrid the behaviour is the same regardless of the "edit mode" one are in: Ie. if you copy a cell, then paste into another cell, the value is correctly paste BUT if you copy and then start editing a cell, make a selection (of a few words to replace) then paste, the whole cell is overwritten. Not as expected!
If you double click in the cell you will see, what looks like, a small checkbox at the far right edge. Click that and a edit window pops up which will then let you do what you want, i.e. select a PORTION of the content and paste to it.