I often find myself doing an interactive search and replace for a string that may be either a whole word or a partial word, such as changing bar to jaz and also foo_bar_blah to foo_jaz_blah.
So I can't just do cw. But if I do for example c/_<CR> then I've lost bar as the search pattern. How can I change just the current instance of the search pattern and retain that search pattern?
To change the current match (or next match, if the cursor is not on a match) use cgn or gnc. Alone, gn visually selects the match.
Here is the answer I finally found after sifting through the vimdoc site, specifically the chapter on patterns scrolling a few paragraphs below the section labeled *search-offset*
An example of how to search for matches with a pattern and change the match with another word: /foo<CR> find "foo" c//e change until end of match bar<Esc> type replacement //<CR> go to start of next match c//e change until end of match beep<Esc> type another replacement etc. The important thing is NOT to use n to move to the next instance, because doing so will put you at the END of that instance. You must use //<CR> as shown. You can use . as usual if you just want to repeat the previous c//e change.
c/_<cr>is perhaps better writtenct_.