It requires to transfer the token license to new device via support ticket, and also need to re-activate the token in user side again. Below are the details KB from Fortinet:
By design, FortiTokens (except the hardware FortiToken-211 and FortiToken-300 series) are always linked to the serial number of the unit on which they are activated.
The entire process boils down roughly to these steps:
1) Migrate the licence/token in Fortinet systems (this is done via a support ticket).
2) Delete the tokens on the old unit.
3) Migrate any user accounts from the old unit to the new unit as appropriate.
4) Activate the tokens on the new unit (add the hardware tokens/supply the mobile token licence activation code).
5) Assign tokens to users again (for mobile tokens, it needs to be activated in the app again).
FortiToken migration need to be preparation