Considering your requirements, I do not think configurable products is in fact what you are after.
You just want to flag an item as 'rented'.
You do not want / or have a seperate line item for the rentable product, as opposed to non-rentable. It is teh same product, same sku.
I suspect you are not 100% familiar with what configurable products are, and when to use them. This will help you:
In my opinion, you want to simply use custom option, and have no need to go change your entire catalog to configurables. (unless you intend to sell a separate sku'd item for the choices)
I have not used custom options in a long time, but I do think they alter the price of the product.
Surely you have a test environment where you can actually go test this? and see if it does?