Is there a way to lock down cards so members can't modify/add/archive or delete?

[Chelsea Peitz]

I only want members to be able to view but not change anything 

[Arnau Martí]

If you own a bussiness class acount you can add Observers to your board. 
With free or gold acount i think the only way is to do a public board. 
Keep in mind that public boards are scaned by google bots.

[Chelsea Peitz]

If I upgrade to business acct and I already have members on a board, can I switch them to observers easily?

[Arnau Martí]

Yes, when you click to a member photo in the top of the board, you can see if you can make them ADMIN or NORMAL. So when you are Bussiness you can choose OBSERVER aswell.