I would love to do this too. Hope someone comes forward with some advice.
I have 40/2 CLE and much prefer the 35 'cron ASPH style focus tabs rather than the pressure finger grip thing on there right now. I have a spare tab but cannot seem to work out how to remove the old one. I have jeweller's scewdrivers and locksmith's files available, but otherwise no specialist tools.
I ended up going with a v.3 Cron and sold my Rokkor ;-p I consider their image quality similar but the Cron is a heavier build and has better resale value, not that it matters but it's nice to have the Leica lens if you can afford it....
I'm pretty crude, I thought the 35/2.5 VC was pretty much indistinguishable too, and it was new.
I just saw this. Looks like too late.
I replaced a tab from a 50mm Elcan.
You don't unscrew it from the front, but remove a retaining ring from the back and the whole lens head comes off. Then you have access to the screw that holds the tab.
But often, every lens is slightly different. I really need to have it in my hands to see how to take it apart.
I just stumbled over this old thread and thought I should update it...
About 8 months ago I pushed with regular but firm pressure on the old focus tab and found that it very (surprisingly) easily popped off the tiny screw holding it to the lens. So it turns out I didn't have to take the lens apart to make this modification after all. I then placed some Araldite (2-component epoxy glue) that I had premixed onto the lens screw and the new 35 cron ASPH thumb focus tab. I held the tiny screw down away from the lens barrell and applied the new focus tab. It has stayed in place ever since. There are no scratching noises or increases in friction that would indicate the screw projecting back into the lens barrell. The lens still works perfectly optically, but the handling has been transformed. I can now focus comfortably and quickly with a single finger.
I recommend this ergonomic upgrade to anyone not shy about modifying an old "user" lens. I had already filed down the mount to enable automatic selection of the 35mm frame lines on an M9.
What a fantastic beautiful little lens this is - my favourite normal lens by far!
Thanks for updating this thread, i'm going to be doing this immediately once my 40 Cron arrives.. any photos and or idea where you got the ASPH tab too? I have some Heavy Star V3 Cron tabs but am curious how a ASPH tab looks.