To be real sure, I tried with a test disk, and indeed, the first thing the "reinstall" option does is to reformat the existing ext4 partition. No luck.
So I'm afraid the "upgrade in-place to 64-bit" solution Nick suggested doesn't exist anymore in 14.04.
And BTW, Canonical, I hardly see the point for the installer having both the so-called "reinstall" option and the "Erase disk and install" one. Both seem to do about the same thing.