you did the right thing for now dude. I got screwed when i upgraded last week and was told that I could keep my vmng300 only for them to swap it over anyway. The VMNG300 can handle 100mbit without breaking a sweat but they are just being stubborn and refusing to use something that works. What's even better is that they lie to you about it. I had a dude from tech support try to tell me the other day that the reason why they needed to swap the modems over is because you can only get 20mbits per docsis channel and 4 channels on the vmng300 = 80mbit but using all 8 channels on the shub = 160mbits. I did laugh before I corrected him.
So far (touch wood) I haven't had any problems with the shub in modem mode but I am waiting for it. Tbh I wouldn't upgrade for another year or so once they have released enough revisions of firmware that they have fixed all the problems, or decided to knock it on the head and go with something better (Ambit VMNG400
)