My take:
Gates for Lewis - Improvement, though probably a regression in replacing Gates on the other side. Probably a wash unless Knevel really comes on.
DTs - Probably a bit of a regression in the short term, though VV was slowed by injuries. Long-term I think the Davis twins will be very near the equals of Collins so improvement down the road.
Anyone for Cockrell - Should be a noticeable improvement. I haven't been a fan of Cockrell for two years. I don't think he really *made* many plays and definitely had his share of screw ups. I would think any of those three will be an improvement.