So I’ve decided to make the switch to 1.75mm filament due to availability and the fact that I plan on switching to a direct drive extruder system. What do you think is the best thing to do with my 3mm stuff now? Make a nozzle that extrudes 1.75mm from 3m on the old hot end? Should I just sell off the rest of my filament?
Use it until you run out, then swap.
While there may be more 1.75 mm suppliers, I suspect that most suppliers that matter will continue to sell 3 and 1.75 (especially because I don’t expect Ultimaker to give up their 3mm Bowden any time soon). I think the money you spend to convert to 1.75 will not be offset by any savings going away from 3. Moreover, flexible filament are easier to work with in 3 mm.
Extrusion of 1.75 from your old 3mm stuff may reduce the quality of the material, it may not be much, but I wouldn’t do it.
I drilled out a .5mm nozzle to 1.7mm and extruded the 3mm off the end of the table and had 1.75mm filament. It’s not perfect but usable and a nice little hack 
Buying the extrudrboard and doubling up on extruders is an option too. That way I would still be able to have the 3mm benefits for some exotic filaments, and I could use the 1.75 for general one color printing.
I have both as it seems when I want to buy filament from a particular vendor they are sold out of the color/size I want so I can just get the other size in the right color.
It’s irritating to have to spend 10 minutes to swap out the hot end, and remember the right setting profile in slic3r, but not really that bad.