Virtuemart error 500

I have had many problems with virtuemart on this site.
I read in another post in this forum that this would be not the best template for virtuemart, but I have everything enabled and I can not change it now.
In Virtuemart forum told me it may be a problem with the template, because it shows me a 500 error when I click "add to cart" on any product.
I changed for a moment the template "beez" for to test, and with "beez" the site show me the content of the cart, but with "add to cart" show me the same error 500.
I must say that this did not happen before and that the problems were others, such as the shopping cart did not carry the sum of the products.
But now, I've updated joomla from 2.5.14 version to version 2.5.17 and Virtuemart upgraded to version 2.0.26. Maybe I have to to update the template too?
Question: Should I update the template to adjust to joomla and VM? and if so, I update it like any other extension manager extensions? (I have many modified files, not only override.css) is this a possible solution?
Or: Is there another solution that you already know?

Hi

Noone reported an error 500 after entering cart to me, so I cannot say where is it comming from. Please update template to the latest version to exclude the case that problem is caused by modifications, but keep Your version as a backup, so it will be easy to get back to it.

Than show me Your site.

A technical question: How do I get the new version of the template does not overwrite the old one?

Hi

I'm afraid it's not possible. Fixes can only be achieved thanks to replacing the files. Please try to separate all changes so You will be able to move it separately from a backup after update. We have prepared override.css file for such purpose.