hy, you don't need to migrate the client agents - but new servers will work only on linux!!
I would recommend that you first install server 2.1.2
The you import the mysql dump from the windows version.
Afterwards you should upgrade to 2.6 with all the Major releases 2.2, 2.3 etc.
Server 2.1.2 looks like 2.0.5 , but with version 2.2 you get a new theme.
keep in mind that some custom fields are not migrated if you step up to 2.3 - but in the latest 2.6 version this should be solved.
It is a lot of work i know - but if you go directly to 2.6 perhaps some database changes aren't done and you end up in a not working Server.
GLPI should also be upgraded - but you will have a lot of work - because every upgrade step has its own problems (php error, password errors etc.)
First of all get the OCS Server 2.1.2 running on Linux with the old database
Kind regards sokatra