I read all about BFG Edition and the censorship issues. Also, that all older doom titles that are bundled inside it, are censored, as well.

I got all Doom titles, before BFG Edition arrived in here. The older, standalone ones; are they censored, too?

Also, IF i buy the BFG Edition, will its old Dooms replace the ones i already own and become censored?

in what way are they censored?

The medpacs are no longer having the red crosses and instead they have a pill, that happened because red cross organization has make the red cross symbol copyrighted.
Also the 2 secret levels of Doom II has removed any imagery of nazis in order to release the same version on germany.

And Doom 3 had stylistic changes for human enemies to remind of human features less.

Plus the whole gameplay element of flashlight/dark areas.