Hi Chickpeas ,
Apologies for any inconvenience caused.
I can't know which product you are changing from "simple" to "variable" from your words. Are you changing a BOM simple to a BOM variable (e.g. Raw material to Variable Ram material) or the parent product with linked BOMs?
Anyway, please note that in each case, you can only manage stock at the variation level, never at the variable level.
You can only link BOMs to variations, and only stock in BOM variations will be available to link to a product.
Best Regards,
José Andrés