It's not as easy as just bolting some rollers onto your trailer you need to have the right equalisers and height of rollers so the boat sits properly on the trailer so it rides and loads properly.
Also I guess you don't have swing bars with your bunks these are important as when you begin to load your boat the swing bar tilts to allow the boat to roll on at a shallower depth if you do not have one you will find that you will just end up with the trailer just as deep as you had with the bunks.
You probably want something like this for the stern depending on what width your trailer is will determine the swing bar width
And then something like these for the front of the trailer.
And probably you will need some keel rollers depending on how deep your keel sits on the trailer.
Extreme marine do all there steel manufacture in there warehouse at southampton and get it galvanised locally as well if there is a place that could sort you out with getting it right I would suggest it these guys.