If you do do a retro fit, can you see if high beam projectors can be made to fit? Is the projector housing a 50mm size in the lights? Hella makes a 50mm and 90mm set. It would be cool if the low beams are 50mm and you could fit the 90mm in the high beam set.
Well, there's good news and bad news. I like to start off with bad news so.....
The low beams are 50mm and Hella charges twice as much for them than the 90mm.

I asked why, and they said it's the newest technology(basically, smaller design). I would like to see someone else's 50mm lamp to see if it's worth the upgrade. They did try to convince me that they both perform the same but then I told them that I've heard 90mm give more light and then they fessed up and said that the 90mm will give out more light. So, if I did do a retro-fit, it would most likely be for the 50mm HID's but now we're talking talking a $1000!
The good news is if the 50mm do perform just like the 90mm but with slightly less light, according to the diagram, it is as well as the HID version a direct fit for this housing and should be an easy retro-fit. But if your determined to upgrade to 90mm, the bracket Hella sells should fit with a bit of modding since it only takes 3 screws to mount the projectors but you will have to cut out the bezel trim that's there for the 50mm. It shouldn't be a problem if they make a hole saw 3 3/8" to cut it out. The high beams are already cutout for 90mm since it doesn't have the bezel trim. I'm gonna see if I can find a set of used HID's at a decent price and go that route.
Here's the bezel trim that will need to be cut out to fit 90mm for the low beam. High beam doesn't have the bezel.
50mm Projector. Has the exact dimensions as the Hella's.
Now I know why my light isn't a straight horizontal beam. The plate is cut out that way. I actually like that because it allows me to see pedestrians or objects on the right much better.
Here's is the 90mm beam off my E350. Not HID BTW.