Change from PayPal Standard to PayPal Express

[DrByte]

So, Is the ZC announcement stating not to use Standard at all and switch *all* PayPal payments to PayPal Express? If so, why?

Because MANY shopowners encounter significant problems due to the way in which PayPal has implemented the Website Payments Standard product, where customers must manually click to return to your store before the order is sent to your store, and even then your store must still wait for PayPal's server to tell *your* server of the order ... all in the background, which needs to happen in the split second between when the customer clicks to return to the store and before your store's checkout-completion page is displayed.
There are many merchants using hosting companies where all those processes don't work well.
We recommend against using WPS because that approach was built by PayPal many years ago just as a "bolt on" to allow advanced database-driven stores like Zen Cart to allow PayPal payments. But, it's full of problems.
Using the more contemporary Express Checkout product allows your customers to get instant immediate order completions, with your store never having to wait for PayPal's server to tell your server about an order in the background, and thus customers get immediate accurate indication of their completed order, immediate access to downloadable products, and no more "lost" orders when communications between servers suddenly don't work properly either due to firewall changes by the hosting company or even just backlogs of transactions on PayPal's side.
Is that enough reason? There's lots more if you need more ... and they're discussed all over the forums. This "recommendation" is NOT new. We've been telling people this for years.

My understanding is that Express requires the customer to already have a PayPal account and Standard does not. If this is correct why would I want to change?

Where did you ever get that idea? It's not true.
https://www.zen-cart.com/tutorials/index.php?article=28