Passcode

If you don't remember the correct passcode for your iPhone XS, the only way to regain access to it would be to restore it. That, unfortunately, will completely erase it rendering it a bit useless for migrating any data.

However, Apple provides you with three methods to set up your new iPhone. See if any of the following support articles for the details:

• Set up your iPhone, iPad, or iPod touch - Apple Support
• Transfer data from your previous iOS or iPadOS device to your new iPhone, iPad, or iPod touch - Apple Support
• Use iTunes or the Finder to transfer data from your previous iOS device to your new iPhone, iPad, or iPod touch - Apple Support

If you are asked for an old Passcode during setup this is for data that is encrypted end to end in iCloud

Some more info here

iCloud security overview - Apple Support

The important thing to note for you is. the Passcode might be an old passcode used when keychain was turned on even on an older device. And that you can continue even if you dont have this passcode, you wont lose all the data you have in iCloud, only the data that is end to end encrypted, you can see what that data is in the above article.

You will not lose iCloud photos for example.

Good luck!

Oh, I can add if you still have the old iPhone available with everything on that this is still very much solvable without losing any data at all, but the data you would lose worst case is the data that is end-to-end encrypted

Oh I see I missed a digit when copying in the link

Here is the correct link:

iCloud security overview - Apple Support

I've updated the original post too