Could it be that the installer doesn't calculate the free space properly?
I have 168 MB free on my 2gb drive and the old OOO folder is 227mb. The new 2.4 installer says it needs 240.4mb of free space (german version). But as its overwriting a lot of the old OOO files, it should work anyhow but the installer wont let me install because it says it needs 240 mb and I only have 168mb.
Shouldn't it work?
Or am I understanding something wrong?

You're right. It doesn't account if the program is already installed. John: Do you have a solution for this?
It should still work. The space needed is automatic but the ability to install it should still be there as there's a second manual calculation behind the scenes that allows you to upgrade when space is low.
Sometimes, the impossible can become possible, if you're awesome!
It doesn't seem to be working for me. I tried the same situation as Simeon, no luck.