Hi,
I will have to accept that it can't easily be fixed
No, you just have to ask the support of the template to fix that. When you report us a problem in HikaShop caused by something used in HikaShop that has changed in Joomla, we update HikaShop to make sure it works fine. That's the same here. It's up to the template provider to update its template and provide a new version for you to get the problem fixed. So for you, it should be "easy" as you just need to notify them about the problem so that they can add the necessary patch.
Regarding the features you're missing, it's not much as we try as much as possible to reduce the possibility of a conflict with customizations so that people can update with as less trouble as possible.
Looking at the changelog you might be missing:
- We replaced the effects (fade, slider, etc) on category listings from javascript to css3 code
But it should still work even with the overrides so you wouldn't see the change.
- We added new restrictions to display item, order and product custom fields so that you have a better control on where the fields are displayed on your website.
- The microdata attributes and tags are now directly in the views of the product page so that it is less resources hungry and will work with any template in the future while the previous method, through a system plugin could only work if the views were not too much modified by the Joomla template.
- We added a new quantity input dropdown type to let the customer select the quantity based on the min quantities of the prices of the product.
- The current category image on the products and categories listing now has a title and an alt tag for better SEO.
- We now resize all the divs in a categories or products listing to the same height for a more homogeneous interface
Remember also that the 2.5.0 is one year and a half old and that the only feature which might break something in a vies upon update og HikaShop since then is the microdata feature that we had to do to make it easier for template providers to freely override the product page layout without breaking the microdata.
I can't answer to the question on what the template developer means when is said that as I don't know. I can only try to guess. Maybe they've updated the CSS specific to HikaShop ? The views should also be updated.
Also, if nothing was customized in the view files, they shouldn't include the overrides. That way, it would reduce the amount of potential problems.
And finally, instead of overriding the view file with a full copy of the view, they should instead try as much as possible to use CSS and use the default views with str_replace on the generated code from the original view files. That way, they would avoid potential breaks and allow for future features to be usable without much issues.
But again, that's up to the developers of the template to do that.