Options 1. Re-use the existing cans. The eyeball trim is typically a separate replaceable piece. From your earlier descriptions, it appears that the light fixtures use A19 type bulbs with the screw in base (Edison). It might be possible to get trim pieces + bulb. That way, you save on the labor required to remove and re-install cans. 2. Replace the can. If you have access to the upper side, removing the can should be relatively easy (though not necessarily pleasant). Otherwise, a large portion of the can should be removable from below. In this case, enlarging the hole is less work than attempting to shrink the hole. Assuming that you are not subject to energy efficiency laws such as Title 24, standard edison base retrofit cans are usable. You may want to use the ECO-575.

Thanks David. I was referring to using a BR30 bulb (which is design to reflect the light downward rather than a standard omnidirectional bulb). What is the advantage of the kit over a BR30 bulb? Does laying the LEDs out on the area under the diffuser make them last longer, provide more light, etc? With the rebate they were only $10 so I bought 9 kits (1 as a spare) but I didn't want to open them up until I understood why they were better than just replacing the bulb. Since the retrofit bulbs have their own trim, my concern is what happens if one goes out in two years and that model isn't made any more? We have 4 Cree can retrofits in the kitchen and one went out after a year or so but they were still available. I would think a mismatch wouldn't be as noticeable if you just swapped to LED BR30 bulbs and kept the original can and trim. Either way you have to worry about mismatched color temps etc. But different trim seems like it would stand out. Then again, my husband nailed a scrap piece of sheetrock over a hole he made when he slipped and put his foot through the kitchen ceiling on his way back from fixing a frozen pipe that ruptured. It was there for over 6 mos and no one ever noticed it!

We used 4" recessed in all of our 8' ceilings and are happy with it. I would also add that to the degree you get worse diffusion with a 4" light, it really doesn't matter that much as this is a hallway and you aren't spending time there - just passing through.

Can you post a photo of the trim? Some trims are really standard- they can be replaced for $10-$18 at home depot plus $5-8 for a good bulb. Sometimes they are not standard but can be fixed with a can of white spray paint. Then other cans have trims that are integrated and can't be removed at all... Generally working with what you have will be cheapest as it doesn't require an electrician to replace the housing in the ceiling. To me LED bulbs have a better light output than the LED retrofit kits, others will disagree. Make sure to watch the color temperature (I like 2700K) and you will need to replace the dimmers as well.