I understand what you actually want to do is a migration, in order to preserver current data: https://help.hcltechsw.com/bigfix/11.0/platform/Platform/Config/r_mgrt_server.html
Since BF 9.5.6 is very old, you need to apply multiple product upgrades in order to move up to v11: upgrade to latest 9.5, then you can upgrade to latest v10 and finally you can have your v11 infrastructure.
https://help.hcltechsw.com/bigfix/10.0/platform/Platform/Installation/c_upgrade_paths.html
https://help.hcltechsw.com/bigfix/11.0/platform/Platform/Installation/c_upgrade_paths.html
Have a look at this post as well: BigFix v11 or v11.0.1 issues