For a case like that, I'll use rectangular block mode--it's really fast and intuitive, so I'll usually pick that over :s/a/b/g if it's applicable.
The whole thing would be something like <ctrl-v>5jecs<esc>. Ctrl-v starts rectangular block mode, go down 5 lines and to the end of the word, change selection to "s", return to normal mode.
The whole thing would be something like <ctrl-v>5jecs<esc>. Ctrl-v starts rectangular block mode, go down 5 lines and to the end of the word, change selection to "s", return to normal mode.