Yes, if you used the pro upgrader to start a free 30 day trial and want to roll back to the free version after your 30 day free trial has expired, this is possible without data loss.
Since Maps Marker Pro v4.0 which features a complete rewrite and does not share any codebase with Leaflet Maps Marker anymore, a rollback from free to pro is not possible anymore as it was possible before with Maps Marker Pro v3.1.1. As both plugins use different database tables, you can switch between those two versions at any time though – changes in pro version will not be reflected in the free version anymore though since Maps Marker Pro v4.0.