To paraphrase my comment below, do nothing on the HE hub, select the existing mobile device in the Hubitat App on your new phone when prompted to when connecting to your HE hub. If you use GeoFencing for presence, make sure you set this up on the mobile App on your new phone.
The main use of the "mobile device" on the Hubitat hub itself is to receive GeoFencing updates, i.e are you arriving or leaving "home", plus sending notifications to the mobile device. There's probably some other one's, but I'm sure more learned people than me can add to the list, but for me these are the key ones.
EDIT - I also do not see any direct link to the mobile device recorded in the device details on the HE hub. The association between your actual mobile and the HE virtual device, from what I have witnessed, is more about what device you select when connecting, not something recorded and enforced by the hub, if that makes sense.
For that reason, the mobile device on HE is just an intermediary of sorts for automations that use / do those two things, trigger on GeoFencing-based presence and/or send notifications. But the HE mobile device, from what I understand, is not really tied to the actual device, so there is nothing stopping you (I think) from selecting the existing device you created for your A13 when connecting to your HE hub.
I expect when you attempt to connect to your Hub via the Hubitat mobile App on your A15 you should be prompted to select from the list of mobile devices on the HE hub, at which point you can, if you want, select the existing A13 device. If you choose to do this, you may, depending on the current name of the device, want to change it's name through the HE Admin Web UI.
By going down this route (selecting the existing mobile device when connecting), I would expect you should not need to make any significant changes to any automations that previously relied on your A13 mobile.
