Can I host programs on Djinn that have been installed before Djinn, or can progs only be hosted on Djinnthe moment they will be installed?

You can only host cards on other cards during the installation. You cannot "move" programs onto a host card (with the usual caveat "unless a card ability says otherwise").

Currently the only way (I think) to re-host a card in this fashion is to Scavenge, trash it, then host when you reinstall.

Thanks for the answers, guys!