Thanks for your interest! VNgen and Edge VN are fundamentally different products. VNgen launched at a discount to give Edge VN users a chance to upgrade, but there's no special discount after that 140 day period. Future sales are a possibility, though. :)
I'm always keeping an eye on GameMaker updates to see what I can take advantage of in VNgen. Version 1.0.8 is well under way and might just be the biggest update yet. Stay tuned!