Debian :: Changing Features Of A Package After Installation?

I was recently pushed back towards linux after spending nearly a decade in a windows only environment and I'm finding myself struggling. The reason for the change is that we purchased Kaspersky and the mail gateway is only available under linux. They advised Debian and Exim.

The crux of the problem is that I have a working debian server running with exim4 and kaspersky to act as a gateway mail server but it's not checking inbound mail against the internal servers address list. For this I need to add ldap support to the exim package but I'm not sure how to do this - it seems silly that I should have to start over at this stage.

Full details:= I didn't have too much trouble installing and configuring a debian 64bit server for our network, adding exim4 and getting it all set up to work, then several weeks of hot fix releases and "oh, it shouldn't do that" passed as we attempted to get the Kaspersky software operating properly. During that time I learned that the kaspersky software does not provide a function to check inbound emails against the internal server. Previously we did this with an LDAP lookup under the Sophos Puremessage software that Kaspersky replaced. Without this function the MTA sends out a lot of backscatter that results in the server being put on blocklists and that effects business. Kaspersky tell me this is something I need to set the MTA software (exim4) to do so I've looked that up and it appears that step 1 is to install exim with ldap support... but I've already installed and configured exim without ldap support. So I'm not sure if I have to start over by removing exim, install over the top or what. Also if I do add ldap support or install exim-heavy over what I have will I need to reconfigure everything I've done so far?

as I said, it's been 10 years since I used linux and even then it was fairly basic sendmail and apache stuff.