We have a Version 1.3.8a site that has been trading since 2008 and we have been told by an SEO company that we have to upgrade to 1.5.4 otherwise an SEO campaign will not be successful, is this true?

We are reluctant to upgrade as the site has a custom template which has been further customised and dropdown navigation and other changes done so an upgrade would be virtually a rebuild and will be very costly with downtime as well.

Windski