Update: for others who may be looking to do this also. I only recently realized that there are two sections on this forum, for pre 2006 vans and the newer 2007 van style. So note this is for a 2008 van.
I ended up purchasing a Kenwood 6.5, 2 way speaker component set which was good value. Door speaker mounting is covered well elsewhere, but the tweeter dash speaker replacement is not.
Yes, you can just pry up the speaker grille cover and remove the current speakers. It takes some pulling and twisting with pliers and a screw driver and of course will destroy them (but they are useless to save anyway). The speaker wire is long enough to allow you to work with them easily. At this time, you could just sit some new small tweeters in their place and be done. However, mine were a little larger and I ended up mounting them in the speaker cover (cutting a small hole in the top) so they are now aimed more towards me. The issue was the speaker grille would not quite sit back down, and rather than hack at the plastic mount below to make room, I removed the dash section they are mounted to. It's a hard angle to work from anyway, so it's much easier to just get that dash section out
1. Remove the surround around the stereo and climate control etc. It just pops off easily.
2. Remove the air vent section to the side of the stereo (lighter gray piece). There are a couple of screws that are easily visible, and then it just pops off.
3. Remove the A pillar that runs down the side of the windshield. Yes it needs to come off! Pry it carefully from the top, and look for the small tab that sits in a housing at the very bottom! So start top to bottom.
4. This reveals a screw that holds the dash. Remove this and the dash that holds the tweeter will come up. Now you can do just about anything to the tweeter mount and put in what you need.
I did also disconnect the center dash speaker as others suggested, and left the speaker in place.
Hope this helps others looking to do this. It really is an easy job. Just grab some trim tools from a parts store and use those, then you will find the trim components are really easy to pull off on a Sprinter, and click back on.