Now, do you have anything to back that up with? I am really interested if you do. I have not made up my mind as to what I am going to do yet.
A guard is something I have been thinking about too. If I could add a pork chop style guard and spring load it, then I might consider keeping the square head. Then if you hand slipped when pushing a board through you wouldn't end up missing a few fingers..
In some ways Jeff I think I do have some stuff to back up what I am saying. Maybe not hard evidence, but I did work in a Railroad Engine House that was 140 years old. Inside we had machine tools that were just as old. When things got slow we used to go out and sand blast and paint these old tools up with whatever color we had. For that reason I never put much stock in changing paint colors. Companies...then and now...did/ do it all the time.
Changing bearings though would be different. You are really not rebuilding the jointer, you are upgrading it, or updating it. Since the cutterhead is the heart of the jointer, you could not help but change its value. Just as if you rebuilt an old car, leaving the orginal engine in place and rebuilding it would make the hot rod more valuable then dropping in a new Chevy engine. I am sure any old iron collector would be dissapointed to learn that the old jointer "used to have babbitt bearings on it".
I am not saying its right or wrong, its your jointer, but if it was mine, I would first try building a guard for it. On my first pass, I would take every precaution known to man, then see how the square cutterhead works. If it still scares you, or grabs the board, or whatever, then I might change it out.
I have a feeling that square cutterheads have a bad rap that might not be so well deserved. I aliken it to the RAS thread. I can see down the road where the RAS is given a doomsday title, when the machine is not that unsafe.
In any case, both round and square cutters can bite, so I am thinking that maybe a guard would have saved fingers in years past. There are so few squareheaded jointers around, it just saddens me to see another go.