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.

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.

Thanks Lawrence. I have also seen a lot of 600 lumens, but also a few 900 lumens. I compared a ton of different brands, actually purchased and installed them side by side for comparison. The Commercial Electric black baffle low glare is by far the lowest glare from almost any angle, but as soon as you can see the bulb, it is blinding (the LED bulb is deeply recessed, but not covered by any kind of shield). Of all the others, the Juno brand retrofit kits were hands down the best of the bunch, you could look directly at them vs. many brands with either no baffle or really weak baffles and pretty bad glare. Just an FYI. Just having trouble deciding to go 900 with the ability to dim, or just stick with my original thought that 600 lumens, even if it doesn't meet the "optimal" brightness level, is a better option. I guess I feel like at 900 I might just have the dimmers down all the time.

When I switched to LED's in the basement, I removed the remodel cans and used a canless integrated LED similar to these: https://www.homedepot.com/p/Halo-HLB-4-in-White-Round-Integrated-LED-Recessed-Light-Direct-Mount-Kit-with-Selectable-CCT-2700K-5000K-No-Can-Needed-HLB4069FS1EMWR/306050711 Prior to installing them, I reinforced the back side of the drywall because I have read that removing the lights can sometimes damage the drywall. I cut squares of thin wood panelling, cut the hole and then cut the square with the hole in half. I glued each half to the back side of the drywall with construction adhesive. Lightly clamped it for a few hours, removed the clamps and installed the lights. There are some good reasons not to use integrated fixtures but I was OK with the trade-offs.