How to get my LR mobile photos from my old to my new Adobe account?

Only way is to download them all when logged onto the old account (select all, export, select file format original+settings) let it download. Log out of the old account and into the new account and import the just downloaded images into the new account. Depeniding on how many images you have this typically needs a lot of harddisk space.

You can also use Adobe's Lightroom account downloader app which is available here: https://lightroom.adobe.com/lightroom-downloader.  This will do the same thing as selecting all and exporting as originals but is much easier to use.