Re: how to switch from rollers to bunks
Yes, bunks do generally give better support unless you have a poorly-built bunk trailer or an above-average number of rollers. With that being said, most boats do not suffer hull damage from sitting on rollers; however, if you trailer often, or over rough roads, or carry a lot of gear in your boat while it is on the trailer, then the chances for hull damage increase.
Rollers have their advantages also, as pointed out in posts above. But, there is no reason you can't have both!
Switch the back 1/3 to 1/2 of the trailer rollers to bunks. The bunks will support the transom and engine, which are usually the heavier parts of the boat. The bunks will also help secure the boat on the trailer. During launching, the boat will float up off the bunks, allowing the rollers to do their job at the front of the trailer. You get the best of both worlds.
Use any good rollers that you remove from the back of the trailer to replace bad rollers at the front of the trailer. For the bunks, I'd recommend a good hardwood, like white oak. Yes, it's more expensive, but it will last longer. I generally stay away from the PT stuff since it is pretty corrosive nowadays. Cypress may also be a good option if you can find it.