There are a few ways to convert simple products into configurable in Magento - and if you only need to convert a small number AND you are pretty tech-savvy your best bet might be to make a small edit to the database.
Here are 4 simple steps to take.
- Inside your database find your catalog_product_entity table, locate the product you wish to change and replace the type_id column for your product from "simple" to "configurable"
- Go back into your Magento admin Catalog > Manage Products and find your product in here - you should now see that it is a configurable product. Click on it and you should be asked which attributes to make your child products with. Choose your attribute and continue.
- On the edit product page - click "Save and Continue Edit". This is important - as this will do a few things in the background to enable you to add child products.
- Once your product is saved you can now go ahead and create your associated products - these should now save correctly
If you forget to perform step number 3 - the first time you try to create an associated product it will bug-out. Especially when you try to create a second one. Don't worry! Just click Save and Continue and remove the broken products you just created back on the manage product page.
So there you have it - one small database change and the rest in CMS. Obviously this might become tedious if you have thousands which is where import/export spreadsheets come into play!