Moving a Member Site to Substack

Hi,

My client has a member site they would like to move to Substack. Anyone have any experience doing this? What is the process of  transferring the content? The member site contains 8 different pages, 5 of which are blog pages.

PS it's a paid membership - what do they do about members who are already paying and have a plan set up? How do you move this across to a different platform.

Any tips would be much appreciated.

Thanks