easiest way for you is to make a backup of your old database, the 1.2.6 one
then upload the zc_install folder from the 1.3.7 distro into your old store
run the upgrader,
once you have the entire upgrade done then export all the tables that say customer or order in them,

make sure that you use the drop tables

and now ( after you make a backup of the 1.3.7 DB )

run the sql file you created from the 1.2.6

this will drop the tables and upload them with the old information in them,