Sell the shaper. Without being familiar with your particular model,
they usually match all components together when making or designing a
purpose built machine.

So, it was probably never intended to swing a large bit. Bearings may
be too small, could be some torque issues (reulting in final cut finish
issues) for the motor that are generated on the end of a 3" cutter that
the machine never had when spinning its maximum designed (smaller)
size.

Then there is the cutter shaft size, whether or not you will get the
quality of cut you want with an after market adapter on top of the
shaft (are you SURE it mates and lines up perfectly?).

I guess the last thing for me is thinking of making a machine do
something like spinning a cutter that it simply wasn't designed to
do. I wouldn't think of increasing a cutter size on a tool just
because I could because of saftey issues.

Besides, if you can sell the thing for more $$$ than you have in it,
you should take the dough and buy the machine you really need/want.

Just my .02.

Robert