You might want to look into "Bulk Import Export Update with Excel" app.
With that you can export your existing products with variants to Excel, then shuffle your variants as you like them to be, and import them back with "Variant Command" set to "REPLACE", so it will replace those variants with your new ones.
And you import back only the columns you need to update, the products will stay the same.
Let me know if you need help with details, I am the developer of this app.
Hi there Trevor!
Don here from Shopify!
I just wanted to jump in here with some extra info that might save you a little hassle down the line.
With any products imported to Shopify via Oberlo, it is not advisable to duplicate these products or to split them into variants if they're not already so divided.
This is due to the fact that modifying the products like this in your Shopify admin will break the link between the products there and the products as they exist in the Oberlo app, leading to issues with orders being communicated to the app correctly among other things.
You can, however; split products into variants within Oberlo as you suspected; just not after they have been imported into your store.
Hope this clears it up a bit!