So I recognized that the blockchain explorer
https://chainz.cryptoid.info/btx/address.dws?1xxx....
shows a zero balance on the old 1xxx address

when converting the old 1xxx address to the new BTX p2pkh address (2xxx) using the following URL
https://limxtec.github.io/btx-convert/

and then inserting that new address into the blockchain explorer, it shows the correct balance
https://chainz.cryptoid.info/btx/address.dws?2xxx....

this is independent whether you already upgraded your wallet to the new version.

As explained on https://bitcore.cc/important-update-bitcore-public-keys/, the private key does not change, regardless of the wallet version used.