Implications of Changing Apple ID on iPhone
I want to know if I change Apple ID on iPhone, do I loose all the data including contacts, pictures and downloaded apps on iPhone
You will not lose your photos, contacts or apps however you will have to provide the old Apple IDs password every time you update an app downloaded under the old Apple ID. You can prevent this from happening by using the old Apple ID for iTunes & Media Purchases.
If you have Apple Pay setup then you will lose all that and you will have to setup your cards all over again. You will lose all iCloud data however you will have the opportunity to update that on the new iCloud account.
Axel F.