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Add an 'l' to get navels (naves to navels)
Target word no 428: vernal
Scrotum, mmmm...
Change 'l' to 't' to get tavern (vernal to tavern)
Target word no 429: scrotum
Add an 'o' to get coaster (crates to coaster)
Target word no 429: scrotum
Change 'a' to 'm' to get scrotum
Target met!!
New target word no 430: gamut
I thought this would be a 6+ letter change, scro are not common and we're going to have to drop 2 letters as we're going from a 7 letter word to a 5. If you still want it changed, I'll consult my target word list which contains about 200 words.
Change 'a' to 'm' to get scrotum
Target met !
Change 'm' to 'a' to get acetous (costume to acetous)
Target word no 430: league
Drop the 'o' to get castle (lactose to castle)
Target word no 430: league