I run a Joomla 1.5 site that has over 400k articles ( and growing ).

I use a Gavick template that is not responsive as all 1.5 templates are.... problem is that Google wants an upgrade to 3.? but it is impossible to do that without losing all the existing content.

I also want to use a more modern Joomla template than the existing look and feel of the site.

Looking for a solution please

I use and recommend SP Upgrade which is not free but does a great job including migrating users and retaining user IDs which can be important if you want to retain article author information, forum post author information etc.

On larger websites, migrations tools will sometimes time out but SP Upgrade copes quite well with this, resuming the migration on restart until the migration is complete.

See joomla.stackexchange.com/a/19351/120 for the procedure I use with SP Upgrade.

If you only want to transfer the articles and don't care about user account IDs, you can probably use a free tool such as J2XML or similar.

I have been using the free version of JoomShaper Helix Ultimate template lately which is a good choice as it will be supported in Joomla 4.