If I spec passive into minion node, do I need to respawn minion?

If I get a passive skill tree point in minion life/damage, so I need to respawn the minion for it to get the buff?

Spectres and zombies are fine, but I have a expensive animate guardian I want to keep after specing a few points into minion nodes late game.

No!
The same goes for support skill gems, If you remove or add/change support gems to the minion setup or add or remove passive skill points in the tree, you dont need to respawn your minion to apply the changes.