What changed
If you guess another form of a word you already guessed in the same round (same “word family” — e.g. related inflections/derivations the game treats as one family), and the new guess scores higher, we replace your earlier guess with the better one. That upgrade does not use one of your six guesses — we’re not punishing you for picking the “wrong” morph first.
If you try another form and it doesn’t beat your current best in that family, you’ll get a clearer message than a plain “you already guessed that” so it’s obvious what happened.
Why
You shouldn’t have to waste turns just to see whether run or running (or similar) lines up better with the theme — if you find a better fit in the same family, you get credit for the upgrade without the extra cost.