Q: I have changed my Apple ID when does it happen on Ipad?

When you go into settings and logout of your old apple id then login with you new apple id .

Please note, you cannot transfer apps from one id to the other. You could look into family plan if you want.

R

If you have your own iTunes account and your wife her own, when you buy a new iPad can you transfer everything from one to the other?

No, but you can use family sharing.

Set up Family Sharing - Apple Support

You songs and apps are tied to your apple id not to a device. While I have not tried it when you get a new device, you log one with your new apple id.  You get access to everything you have with the old apple id.  You may have to do more.

R