I have had my avocado plant in water for 6 months now on my window sill and it has grown to be about 30cm. However the bottom leaves are hanging and have slight brown spots, the second set of leaves are thinner and slightly curly whilst a third set is currently about to grow out. I assume this is because it is still in water? Will it be able to readjust if I plant it in soil at this point? Be a shame to see it go.
I don't spend any great deal of effort on these, but one or two have landed where I see them. My impression is that loss of lower leaves could be classed as "perfectly normal", especially under less than ideal lighting conditions. I've seen exactly one that really amounted to much, and that was a professor's "mild eccentricity" growing in a 30 gallon pot in an office with extensive windows and the university paying for the heat...
Anyway, I see no reason not to put it in dirt if you want to keep it.
Definitely transition it to soil. Your plant is using up the energy reserves in the large seed. If it runs out, and it doesn't get the proper nutrients, it won't be able to support itself, and it will die. Use a pot that has a diameter not over an inch wider in diameter than the root spread. Use a free draining mix, and keep the plant well watered until it puts out new growth.
One common thing people do (wrong) when transitioning from all water to soil is placing the container into a bowl (or similar) of water, to keep the mix saturated. Do not do that, as it starts anaerobic decomposition of the mix, and is a perfect place for phytophthora to kill off the root system. Keep the mix moist, but not soaking wet.
It will be a shock to the plant, and it will have to put out a different form of root hairs into the mix, after which new growth should show and you should begin fertilizing.