LIES AND SLANDER.
I have a test (SSP) world that was created in the Beta A pack. This test world contains every single block, machine, and item from every single mod included in Beta A. (Alright, I lied. About 98% of items.) I copied the world save over to my dw20 and Mindcrack installation. Both modpacks loaded the world completely fine, no crashes, no blocks turning into something else. Obviously blocks from mods not included in a pack disappeared from the world. The only issue was with Portal Gun - the world blocks (portal spawners, aerial faith plates, cube vents...) disappeared. Portal guns in chests however remained.
So yes, if you're playing the beta mod pack, want to upgrade to 1.4.6 (and get RedPower), and you don't mind not having Mystcraft (Mindcrack) or GregTech and TF (dw20), feel free to upgrade to one of the packs and transfer your map over.
DISCLAIMER: I only tried this on SSP. I haven't tested doing the upgrade with actual connected pipes and running machines, just a bunch of blocks in the world. There are still things that could go wrong (so absolutely make a backup of your world), but if something goes wrong, it won't be ID conflicts.