Every BCH address which starts with "q" has a legacy format too. We have one address with two formats.
I just created a BCH address: qpj6l74xs9g93yuz67lrkqvjpfunr9rewqpuz4pq09
There are some tools that gives you the corresponding BTC address. Here is the bitcoin address (legacy format of that j format address): 1AGg4cofDoMHatd2wCcMuFHUz7WYERq6jh
The j format address is also called "CashAddr" format.
The private key of both of addresses is L3unCBMZZLJxHbJyNtDiAupX8mqBx5xDGESzQ8Kz7869ECsBnWgd.
You can test it. You can use this private key for unlocking the legacy address in bitcoin network and also the CashAddr address in Bitcoin Cash network.
The new format of BCH addresses was created so people don't get into trouble. But there are always some people send BCH to BTC address or vice-versa.
If you send BTC to a cashAddr address you will get an error. But if you send BCH to a BTC address or BTC to legacy format of a BCH address, you won't get any error.
In the case you use a custodial wallet and you don''t access private key of the address, you will lose your fund, unless support of the wallet helps you to recover your fund.