Ask yourself what the Selmer will do for you that you're not getting from the Conn. Play test the Selmer. It's not a slam dunk that you will like the Mk. VII - some folks do and some don't (I don't, but that's beside the point).

I play in a big band next to a Mk VII which is really a great sounding horn. This one has nice action and better intonation than my VI. I still prefer the voice of my horn, but this particular VII is without a doubt a good one. Others I have heard and played on weren't great. Depends on the individual horn. Some late VIs wern't that great either. If your Conn is a good one, I wouldn't be in a hurry to trade it for a VII unless you've played it and it just stole your heart. Is the tranny a good horn that you love to play?