I changed my iPhone. Now i have to sign in to my new iPhone but it requires old iPhone screen lock passcode. Unfortunately I forgot that. Now it says to reset end to end encrypted data. If i reset it what datas would be losing

Resetting encyrpted data was discussed in this thread:

https://discussions.apple.com/thread/8463493

This will apparently remove some but not all data. The process for getting into a device for which you've forgotten the passcode will erase that device completely: If you forgot the passcode on your iPhone, or your iPhone is disabled - Apple Support

Forgotten Lock Screen Passcode or iPhone disabled 

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 ...