Full agree with Marc answer. Obviously you should check what is doing the concurrence or similar products on the market but I guess that you already did it..

My advice is to give grace period of 1-3 months to the current userbase and give them some invite vouchers and make the new subscribers pay (start low as Marc said so they never feel overpaid). Then you'll gather enough metrics to adjust your pricing strategy accordingly without it impacting your userbase (actually some are gonna love you for making them feel special early birds who don't have to pay at the beginning).

Yep, why not, anything to make the UX easier on this would be cool cause I literally couldn't find out by myself about the date permalink trick :) Thanks for your answer

Can't find any easy way to share the link for a todo, I know there is one since @marc posted a link to one of his todo literally three commment on top of this one. Maybe simply add a Copy link to clipboard button (maybe there is one and I'm blind)