I just tried the solution suggested above by Nick (Oct 19'13): Using the Ubuntu 64-bit Live CD to do a 64-bit reinstall.
I used the Ubuntu 14.04.4 Live CD. But if the first option is indeed called a "reinstall", it more precisely appears as:
Erase Ubuntu 14.04.4 LTS and reinstall
Warning: This will delete all your Ubuntu 14.04 programs, documents,
photos, music, and all other files.
The two other options offered by the installer on the Live CD are:
- Install alongside (dual boot option)
- Erase disk and install Ubuntu
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.
Too bad, since such an upgrade-to-64-bit-in-place option could have been real nice!