Can I upgrade my iMac Retina 4K SSD to 1TB SATA

You can however it would be MUCH wiser to simply add an external SSD and use that as your boot drive.. This is lot easier and less expensive. You can find instructions in How to setup an external SSD as your startup disk . The SSD I would recommend is the https://eshop.macsales.com/shop/owc-envoy-ultra. This will be a HUGE upgrade for your 2019.

Yes. It's not a particularly easy process since the iMac is basically glued together so getting the case open and then reassembled later is rather a headache. You'll probably get close to the same performance with a lot less hassle if you just get an external Thunderbolt 3 drive. But if you want to go ahead with the internal swap, Other World Computing (aka OWC and MacSales) offers the drive, and the "fixit" type sites have instructions.

Regards.