If the software you're leaving already holds cards for these customers, those can usually be transferred rather than re-collected one at a time.
How it works
It's a processor-to-processor transfer. Your old payment company sends its stored cards directly to Stripe over an encrypted channel. Card numbers are never downloaded, emailed, or handled by you, by PortaPro, or by anyone in between. Stripe doesn't charge for the transfer.
This is worth understanding, because "export my cards" sounds like a file you download — it isn't, and any vendor offering you a spreadsheet of card numbers is doing something you don't want.
What it needs from you
Two things:
How long it takes
Usually one to three weeks, and nearly all of that is waiting on the old provider. It runs in the background and blocks nothing, so collecting cards by hand in the meantime is never wasted work — start your top customers manually and let the migration catch the rest.
What happens when it lands
Cards are matched to your customers in PortaPro, and autopay follows one rule:
No customer starts being charged who wasn't being charged before. If you want, send a one-time heads-up to the customers who carry over: "we've upgraded billing systems; your card ending 1234 stays on file for your monthly service."
Expired cards
An older list always has some. Most refresh automatically through the card networks' updater service. Any that don't simply decline once and trigger the update-your-card notice, which your customer can fix themselves in a couple of taps.