If you bought a copy of Mastermine on itch.io don't worry, you'll be emailed a Steam key at a time close to the game's release date. You will not be required to purchase the game again.
All your progress will also be moved across automatically after downloading the game via Steam.
Moving to Steam, the biggest platform for PC games comes with a bunch of awesome benefits, with just a few being:
Via Steam, your save file including your store credits, store-bought powerups & skins, etc. will be automatically kept up to date between all of the PCs you play Mastermine on. Save syncing works across all operating systems (Windows, macOS & Linux).
Save syncing doesn't affect save files on your phone (iOS/Android) at the moment, as it is a feature of Steam.
If you previously bought the game on itch.io, the Steam version will pick up and continue to use your previous save file. Just make sure to install the Steam version on the PC you played your itch.io copy on.
When the Steam version of Mastermine is released, the itch.io store page will be wound down and all future updates for the game will release on Steam.
Maintaining one PC version of the game means higher quality updates for everyone.
On Steam release day, the game will be update 1.11.2 - this is the same version as is currently available on itch.io, plus changes relating to supporting Steam features like Achievements. More major updates will be coming after everyone has moved over to Steam.