Create a Sales Order with MTO or MTS

Hi Gurus,

Im trying to define a sales process with 2 kind of item cathegory for the same item

when i create the sales order, the item default is TAN, if there's stock, no problem i can delivery

if not, i would like to change the item cathegory manually and it create a MTO process.

Can you please help me in the SD configuration, all the steps please.

How can i costumize the item, and the SD definition (item, etc..)

its very urgent!!

thanks in advance gurus