Man, I just did a 9 speed conversion on my 3x8 speed commuting #Bicycle that I've had since 2015. Look what time it is, past 1:30 a.m.

I put an Shimano Alivio shifter, and SRAM 12-23 PG-950 9 speed cassette. Put the shifter into gear 1, pulled the cable as tight as I could with some vice grips while clamping it into the derailleur with my free hand. And ... thats it! Bang on, butter smooth shifting into every gear, up and down. Nailed it first time, just with the cable clamp; no barrel adjusting, nothing! The cable might stretch over the next week or two, though. Remains to be seen.

The derailleur is already 9 speed compatible: a Shimano Sora medium cage. Used that for years, because it worked with the small range cassettes I used.

So now I have all the same gears as before, plus a new 16 between the 15 and 17. Yay ...

I only did this because 8 speed is getting hard to find readily available parts for, especially road cassettes with close gear ratios.