Originally Posted by Kapusta

Are you looking to just drop the big or small ring, or go with entirely new ring sizes?

A lot of people have done a conversion because they don't really need their big ring, so they just run the small and middle ring. In fact, before 2x10 came out, 9 MTB 2X cranks were mostly just 3X cranks with the big ring replaced with a bash ring.

If that's what you want to do, you can replace the big ring with a bash ring. BBG makes good ones that are very cheap (like ~$20 shipped): https://www.bbgbashguard.com/

Then turn in the high-limit screw on the front derailleur to keep it from shifting into the big ring position.

Then you shorten the chain using the middle ring as the new "big" ring for sizing the chain length. This step is REALLY important because this is (IMO) one of the real benefits to running a 2x drive-train: A shorter chain which means less chain flopping around and much fewer dropped chains.

I have done this on all my 8 and 9 speed mountain bikes since 2001. I just don't need the big ring on trails. The only place I spin out is either on pavement or really fast, wide open fire roads. It is easier to keep a 2x setup shifting well. I have changed the 32t middle ring to a 34t or 36t on some setups, but even the 32t with an 11t small cog in the back seems to do fine for me. On my current 2x9 I went with a 24t small ring and 36t middle ring.