This is true, but I advise that players be careful.
I noticed in the release patch that despite checking specific equipment and setting the garrison template to low priority for reinforcements and upgrades, I was still seeing Tier 2 and Tier 3 infantry kits in garrison forces.
I need to test it further to see if I'm doing something wrong (like not setting it correctly until after the garrisons get certain equipment), or if the game is just not prioritizing things correctly.
Already given stuff won't get removed. So I think the best way would be to set everything to No Garrision, change your equipment filters, then use garrisions again. Only then they should only take old stuff.