In existing Outlook, use the File >> "Import & Export" tool to create a PST file -- this will contain all folders, messages & contacts.
Then go to "Tools >> Accounts" & export each account as an "IAF" file
Now remove Outlook 32-bit.
When you've finished installing Outlook 64-bit, use File >> Import & Export to import the PST file.
Then go to Tools >> Accounts & import each IAF account file.
Then go to "Tools >> Accounts" & export each account as an "IAF" file
Now remove Outlook 32-bit.
When you've finished installing Outlook 64-bit, use File >> Import & Export to import the PST file.
Then go to Tools >> Accounts & import each IAF account file.