A migration tool upgrading from 32-bit to 64-bit might be a bad idea, since it’s quite advanced, would end up with a mess and is entirely unsupported. 
It might be possible to use the installation option “Reinstall Ubuntu but keep my personal files” on the 64-bit install medium – this might also keep applications too (should they have a i386-equivalent), but I haven’t tried it.
A fresh install is strongly recommended when switching an existing system from 32-bit (i386) to 64-bit (amd64). It’s not technically a 1:1 process.
Be sure to back up your important files and optionally make a system image!
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