You can upgrade from 12 TRU to 24 TRUs but you need to have 2 E1.
I assume that your Concentration factor is 32Kbps(maximum of 12 TRUs in 1 E1).
Also, if the frequency is not that tight, you can also use 900?
If you are using an RBS 2206, thats a dual TRU. 1 TRU is equal to 2 TRU capacity. HTH.