Hi,
Yes, if you changed the weight unit in VM HikaShop couldn't recognize it and defaulted to the main weight unit of HikaShop (the first one in the list of units of the "weight units" option of the configuration).
So in order to avoid that problem, just changing the order of that option before the import would have solved your issue.
Now that it's done, you'll have to create a new mass action via the menu System>Mass actions with a filter value of 1000 (to cover all the products), and an action "update the values" on the column "product_weight_unit" with the value "lb". Then just click on the "process" button to run the mass action and change the weight unit of all the products at once.