When I moved to v4 (I am now on 4.05) it seemed a major challenge for Viart given the mods they'd provided for me over the year - though I have to say that given they were Viart mods I would have thought it would have been much more straightforward.
 
Now I need to understand the route from, say, 4.05 to 4.07. Is it another complete upgrade process and watch out for those mods again - or just overwrites of changed files?
 
Chris

interesting to see any answer to this query.
Viart, please let us know, especially for those who have modifications.

How to upgrade is explained in our manual:
http://www.viart.com/upgrade_step_by_step.html
 
Among other information you can find there the following:
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If your shop has any programming or design modifications we'd recommend you to pay attention to the following tips:
 
* Keep a list of the files that have been modified during your customizations.
* Make a copy of your site in some test folder and perform an upgrade there to ensure that everything works properly.
* Once new files are uploaded look at the files side by side (in other words, compare files that were modified with the old ones).
* Copy over your customized code to the new upgraded files leaving the rest of the new code in place. Save the changes.
* Before editing a file, copy it with the original filename and append "OLD" before the filetype so you can instantly see what's been changed.
* If you are adding or changing message or language files leave the original and add changes to the end.
* Once done comparing all of the pages you're finished with the upgrade of your test site. Now you can migrate the files to your live site.
 
Note: If you're not confident about making a custom upgrade yourself then we suggest contacting our support team for a quote on a custom upgrade service.
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I can also add always make upgrade on the test copy of your site (you should fully recreate your site on the test environment including all files and database settings) and then after you successfully upgraded your test site along with all custom features proceed to upgrading the live site:
* upload all files from test copy over your Live files;
* change the var_definition.php file to link to the Live database;
* click 'System Upgrade' in admin panel;
* apply dump of CMS tables from test copy to the Live site (to save CMS layout changes if you made any).
 
Regards,
ViArt Team

FYI, I did our upgrade to V. 4 earlier this year. I used WinMerge for file comparison, which worked some of the time.
 
We still had a few bumps, but paid Viart to fix some of their previously customized work, and we were able to get everything upgraded and working fine.