ok
2e and 3.5X can EASILY co-exist.
Here are some modifications we use in my campaigns (house rules), that I will continue in 4E...
Vorpal rules follow 3.0E version (on a critical, instead of on a 20)..my players haven't faced and don't have one yet and I would leave it for epic levels anyways, so not a big deal for them.
Upon a critical hit, we compare weapon size, vs target size and use the detailed critical hit system from Combat & Tactics from 2E. It works great.
After casting a combat spell and injuring something; roll a D20. If you roll 20, the spell causes criticals, based on the Spells & Magic detailed critical hit chart in 2E.
For various monsters, I've added back powers that they had in 2E that gave them more flavour but if it makes the monster much more powerful or weaker, I will modify exp accordingly for the encounter.
We play a planescape setting and use that cosmology and just incorporated Eberron, etc into it...easy as hell, due to planescape's freedom.
Planar creatures that Gate (ie. tanar'ri) can gate after they themselves have been gated in ala 2E. This makes perfect sense for their method of combat (attrition). Basically, have changed how I see their summon abilities to follow the 2E version (ie. a balor can gate 1 true tanar'ri each hour instead of 1/day, etc).
Some classes have been modified a bit with a new chart we wrote up, to yield what was more fun. (ie. the wild mage wild chart from 2E was converted to a DC chart which results in a 1 causing a wild surge) and then added in some stuff from the 2E version with the 3.5E prestige class and made a new specialist class. Did the same with Chronomancer.
Spells or items that are not in 3.XE, we convert into it..just changing whatever we need or update spells as we'd prefer them. Our disintegrate, works a bit differently. We target the area it hits based on a locational chart. Then roll save. Upon failure, that body part starts to disintegrate. Then roll another save for the next adjacent part. It makes the spell result 'cooler' as people start to disintegrate in front of them, but more chances to survive it overall since your arm could get vaporized, but then you save before it goes past the shoulder, etc. Also, horrid wilting we've raised back to 1d8 instead of 1d6...small stuff like that.
It's really not hard to combine house rules that you really like. I *KNOW* the experienced players here, especially, will have no trouble finding a way to modify 4E incase it 'removes' something that we totally can't live without
Sanjay