Do I have to transfer foam pads when replacing the back housing?

It would be needed, highly advisable as it will stop the logic board from touching the bare metal and stop it shorting out thus leaving the phone useless and damaged.

There's nothing speacial in replacement the back housing, all You need is patient, some kind of hot air, and remember. STAY ORGANISED, and do not follow everything what You see on YT or some kind of it before you watch it to the end :)