Doesn't look like it'll work, without modifications anyway. The blade thickness on the 940 is 0.115, blade thickness on the 921 is 0.100. The pivot pin is also a smaller diameter on the 921. Washers could correct the blade thickness and the pivot hole could be enlarged (but the drill bit would have to be perfectly centered). Then you have to worry about whether the stop pin and axis bar are going to coincide with the machining on the tang and whether the cutting edge is going to come in contact with the backspacer when you close the knife.
Edit: I was thinking that you wanted to put a 921 blade in 940 scales. But putting a 940 blade in 921 scales would still be a no go, the modifications for trying it would be slightly different than what I explained above, but thats the gist of it anyway.