Piwik can only apply the new geo data to new visitors. If you want it to work for all visitors your site has had, you need to force Piwik to re-process the reports. See here: [piwik.org]
This should work on the condition that your visitors' IP addresses are still fully in the logs, i.e. you did not tell Piwik to shorten them.
This should work on the condition that your visitors' IP addresses are still fully in the logs, i.e. you did not tell Piwik to shorten them.