Follow the same steps for a new install, but make sure to complete the step below so that Klarna is not duplicated in your store's checkout.
In Shopify store admin (under Settings -> Payments), deactivate your current Klarna HPSDK payment method(s), e.g. "Buy now, pay later with Klarna". This step updates your Shopify checkout. Do not delete the current HPSDK 'Klarna Payments' app (under Apps) OR deactivate the Klarna API credentials (in the Klarna Merchant Portal) used for the HPSDK integration, at least not until all previous HPSDK orders are finalized. Klarna orders are automatically updated (i.e. captures, cancels, refunds) in Klarna via the HPSDK Klarna Payments app. Note: only if necessary, orders can be updated manually in the Klarna Merchant Portal, but be careful to not duplicate order updates (as there is a time delay of about 5 minutes for updates from Shopify to Klarna).
As long as your Klarna Merchant ID doesn't change, no changes should be necessary in the Klarna On-site messaging app when you switch from the Klarna HPSDK integration to the new payments app integration.
"Sofort" as a local payment method (within Shopify Payments) OR
"Klarna" as an alterative payment method (separate from Shopify Payments), which can include the Sofort payment option if activated in the merchant's Klarna contract.
Note: if Klarna is activated in a store as both a local payment method (within Shopify Payments) AND also as an alternative payment method (not within Shopify Payments), only the local payment method will appear as a payment method in Shopify checkout.
The merchant's current Sofort project/contract will not be transferable when migrating to a different payment method.