my phone asks for my old passcode(one previously used to unlock the iphone)

Incorrect Process:

Erase All Content and Settings and use a new Passcode after the erase won't work because it will ask you for the old passcode. You have only one option read below

Correct Process

If one had attempted unlocking multiple times with incorrect Lock Screen passcode, or if a message says that the device is disabled, learn what one needs, what one gets, and what is lost.

You need a computer to follow these steps. If you don't have a computer, borrow one from a friend.

Forgot? Click below to reset…

Click->If you forgot the passcode for your iPhone, iPad, or iPod touch, or your ...

Regarding your data, If…

1. You have an iCloud or iTunes latest backup, then there is nothing to worry about. If not read more...
2. You have not backed up or do not have the latest backup, nor imported nor you have synced nor uploaded your data to cloud drives prior to the situation that you are in, then I am sorry, all your data is lost.