Hey Darren,
Thanks for reaching out! Events Calendar Pro does not replace The Events Calendar on sites — if you upgrade to Events Calendar Pro, for example, you do not delete The Events Calendar first then add Events Calendar Pro in its stead.
Events Calendar Pro is an add-on to The Events Calendar. So if you add Events Calendar Pro on your site, you just activate it alongside The Events Calendar, and so all of the event data is totally the same and is not lost or affected in any way.
Please let me know if this helps and if there are any other issues or questions I can try to help with!
— George