ar eyou talking about the firmware or the codeplug, the firmware is the sort of the operating system for the radio, while the code plug is the specific information for frequencies/codes/etc.. the MTS has not been made in several years, so the newest firmware is likely still 5-6 years old. there is no need to upgrade it unless you have a problem that is caused by one of the bugs a later FW resolved.
to directly answer your question, you do not have to purchase a FW upgrade. though you certainly have to pay someone to load it if you don't have the equipment. since they are no longer officially supported, i don't know if you can even get the FW anymore. i am unsure if the firmware could be loaded normally or required the FLASHport rib, never had to update one after FLASHport came out.
EDIT: i just checked MOL and they don't show it anywhere. but then again my subscription to the MTS/MCS software is expired so i won't see updates even if they are old.