Stockable variants VM3 back-compatible

[pcmilhouse]

I've a site with VM 2.6.18 and Joomla 2.5.28. I've more than 2000 product with child products generated through custom field (stockable variant type).

Trying to upgrade to Joomla 3 and VM 3.0.9 but I can't see variants in product details and in administrator page.

How can I upgrade keeping child product generated by stockable variants plugin?

Regards

Paolo