Code 128 barcode set switching

Hi,

I'm encoding a barcode which has a series of digits in the middle
which I'd like to encode using code set C for size reasons.

Is it possible to switch to set C after printing the first few
characters, and then switch back to set B?

Example:

s = 'AB12345678CD'

I'd like to switch after 'AB' and then switch back after '78'.

Would I have to encode the neighbouring digits as one code symbol
myself as well?

Thanks,
Ben.