Dasper I got your email. I am unable to reply to emails at this time but I can recieve them. Since I got up this morning I have not been able to send any. I'm not sure what is going on there. They said on the phone it should be fixed up shortly. Of course that was this morning though.
As to your question. I've never tried it. The pivot will be the same diameter on both and I would think the stop would be also but the location of each in relation to the handle dimension differences from one to the other are not likely to be identical. Therefor I'd be surprised if they did match up. If they did then one blade should pop right in the other. On the longer handle you'd have the shorter blade but on the shorter handle the tip would stick out the end.
This will only work if the folders are identical in everyway but length. If the width is different on the bigger model then it changes the location of the stop. the width of the blade and also would change where the blade positioned itself when opened and when closed in the body of the folder. Of course it goes without saying that the lock cut would likely be in a different place also. There is a chance it could work but the only way to know is to try it.
One thing I do know is that even on knives of the same model you can sometimes run into issues where things work differently. For example I've had two SARK models I did this with. One with a combo edge blade with satin finish and in green G10 scales. The other was with a satin plain edge in black G10 and I wanted the satin plain in my green handles so I swapped them. Turned out the lock that was originally with the plain edge blade had to be swapped with the plain edge blade because it was tuned to that blade and the one for the combo blade overtraveled on the plain edge blade to where it nearly was leaving the blade but locked up tight in the middle on the original combo blade. So it may be that your lock does not function idealy moving the blade like this without the lock and since the lock is different it negates the whole deal for you in this case if that is what happens.
Something tells me that Ernie would tell you he does not recommend this. The best way to find out if its possible, or advisable or if the two models are identical other than length is to discuss it with Meagan to relay it to her dad or someone else in the know there at EKI if she doesn't know off the top of her head and see what they recommend due to your particular situation.
I am sure if the cut for the blades regardless of length is the same in the 2.9" vs the longer one that the rest of the folder would be set up the same also and due to cost factors its a high probability they are the same but again its all a big guess for me to answer your question better than that. Sorry. I hope what I've passed on can help you some. Its the best I can do without having one of each in front of me to say, yes or no.
STR