I'm not sure about expired Road Tax. Normally you can transfer, but to new car (unreg car) only.

E.g:
Scenario 1:
Existing: Myvi - WAA 1001
New Purchase: Bezza
You can transfer WAA 1001 from Myvi to Bezza and buy new number for Myvi.

Scenario 2:
Existing: Myvi - WAA 1001
Existing: Wira - WBB 2002
You CANNOT swap both number.

Scenario 3:
Existing: Myvi - WAA 1001
You think WAA 1001 has no luck and want to change new number. Yes, you can do so but pay a bit more.
1. Buy new and cheapest motorcycle and transfer WAA 1001 to your motorcycle.
2. Your Myvi is now numberless. Buy new number, such as RQ 888.
Result:
Myvi - RQ 888
Motorcycle - WAA 1001