The ITSM plugin is an additional layer above OTRS; It doesn't change
OTRS in great ways, but rather extends it.
So, I don't believe there will be much impact on your existing setup.
Sure enough, you don't have to change anything to your current setup to
be able to install the ITSM package. Though, you'll probably want to
change it to take advantages of the ITSM package's features.
Please, also try to explain to your colleagues that there isn't such a
thing as an "ITIL compliant" software. Some software, in some version,
have been certified to be ITIL compliant for certain aspects of ITIL but
*none of them* has ever been certified for all the ITIL aspects.
BR,
Cyrille