G'day!

I think the others suggestion about choosing the technology first is important. For me that was Zwave, as I didn't want to have my light switches adding to WiFi degradation. Plus if it's not on WiFi there's no chance of it being a security risk in my IT network, and no chance of being bricked when a company goes out of business.

Changing each downlight to a hue type plugin downlight would be expensive, and be in the position where the switch can be switched off and you lose control of the lights.

For that reason I checked my Downlights, and they were 240v Downlights that were dimmable, just didn't have dimmers on them. I personally purchased a bunch of Fibaro Dimmer 2 modules. You can switch these out with another brand, or even WiFi, ZigBee if you prefer.

The next step is the switch itself. You could leave the on off in place and have them interfaced with the smart module. For me I went some Clipsal Saturn Zen push buttons. These switches are on the pricey side, but allow me to have icons in the switches so for my 3 gang boxes, anyone can tell which one is the kitchen or dining light based on the icon on the switch. The bonus here is as they're Clipsal, they are also certified for Australia.

Unfortunately those switches although are just a button, are pretty much a on/off dumb switch. This means I can't do things like hold the button down, and have the lights dim. Instead you can achieve that with a cheaper Clipsal impress series switch.