Unitrunker will display the LCN as soon as it is first used by the system. Depending on the type of system, Unitrunker might be able to determine the frequency given the available information or it may require you to enter the frequency manually.
No because Unitrunker has no way of being able to determine the moment a new LCN is added. Most systems do not transmit that information as the radios in the field generally don't care if there is a new channel. They spend their entire time being directed towards a channel as determined by the site. Rewording my previous answer, the soonest you (and any of the radios on that site) will know that they've added a new LCN is when it is used for the first time.
The exception to radios not caring about the LCN would be a few trunking formats that don't depend on frequency tables (eg. EDACS, LTR). Those radio needs to be reprogrammed with the frequency corresponding to a new LCN. That is certainly information that Unitrunker certainly has no ability of accessing unless it were manually added into Unitrunker by a person reprogramming the radios.