Changing the frog sprites to glenn wouldn't work out so good, Glenn would be a midget.
and when and NPC follows the characters it generally doesn't look right.
Currently it's best as is, waiting for the glenn sprites to be finished, and then putting them in.
It may seem odd at some dialogue points, but it'd be wise to have it be Glenn rather then frog in the dialogues, you don't neccessarily want to go back and end up missing one. Also it's a reminder that Glenn will be playable sometime in the future. It may be very hard to imagine, since this hasn't been accomplished before . But right now we're taking the appropriate course of action.