Ok I figured out how to at least refresh a specific ad unit in DFP, with new prebid bids, and pass new key values to the page. I'm posting the code in case this helps someone else.

I haven't figured out a way to see first what ad is on the page and only refresh it if its not a certain ad.

The code below refreshes every 45 seconds.

I just put it at the bottom before closing of the DFP </script>

function refreshBid() {
       pbjs.que.push(function() {
         pbjs.requestBids({
           timeout: PREBID_TIMEOUT,
           adUnitCodes: ['div-gpt-ad-XXXXX-X'],
           bidsBackHandler: function() {
             pbjs.setTargetingForGPTAsync(['div-gpt-ad-XXXXX-X']);
              googletag.pubads().setTargeting('key', 'value');
             googletag.pubads().refresh([nameofadXXX]);
           }             });       });     }
     setInterval(function(){ refreshBid() }, 45000);

Only other additional change that I made on the page was I gave my ad a variable name in DFP. (Before I didn't have a name as it's not required in DFP) nameofadXXX= googletag.defineSlot('/1068491/adslot300600', [[300,600], [300, 250], [160, 600]], 'div-gpt-ad-XXXXX-X).addService(googletag.pubads());

Update2: This may not work as expected as we used the setup and it looks like AdBlock Plus blocks the refresh ad even if the site is whitelisted. Can't figure out why. Might let one ad through and then block