WPML created a plugin named Polylang to WPML Importer, which imports Polylang to WPML as the plugin suggests. This tool is almost complete and they are looking for sites to test.
If you request the plugin at this page, you will have an opportunity to test it. Apart from it, a developer named kkarpieszuk released the migrate-polylang-to-wpml plugin (Alpha version) which is available at github.
Please be sure to backup your blog's database before you use these plugins.
Just for your reference: WPML 3.4.1 and String Translation 2.4 are reducing ST memory usage by about 70% and execution time by 50%. WooCommerce Multilingual 3.8.1 is cutting the load time of the Products admin screen to about 40% of the current timing.
I once installed and enabled Polylang on this blog but removed it because it affected the site performance and speed. Recently, I installed WPML on other blog. I tested the speed of the blog in which I enabled WPML. The result is surprising: the load time is less than one second.
I installed more plugins and now the load time is about 1 second: https://tools.pingdom.com/#!/cpPNRG/http://iwordpower.com/
The test blog uses one of the cheapest plan, the Essential Plan of iPage.
WPML, premium multilingual plugin for WordPress, shows higher performance than Polylang and affects lower impact on the speed of WordPress blogs. If the official version of Polylang to WPML Importer is released, it will be easier to change Polylang to WPML.