You're not really changing sts in mid row, only when you turn and work on the back. Then you'll be changing from knit to purl sts as you have been all along. For the last pass of entrelac, you need to do triangles which are similar to the triangles that start the project. You need to decrease at the beginning of the purl side on each purl row, purl the last st, join to the picked up sts with a p2tog and then turn and knit. Each knit row becomes shorter by one st as Evapche says, until you're left with k2. You're working with the new color (which is used tp pick up the joining sts).