I had to remove my pine from its pot rather brutally. Very annoyed to lose a pot - but I really didn't have much choice.
While a baked clay substrate that doesn't break down is very nice, it also doesn't compress, so if you have any reverse taper (or a large inner lip) on your pots it can be difficult to repot a tree after a few years.
I did spend quite a bit of time trying to get it out and eventually gave up and used the hammer. A light tap eventually got it out.
The straight line on the pottery shard illustrates the inner curvature of the pot. I do check all the new pots I buy now to make sure that there will be no difficulties getting the bonsai out of them.
In the end the roots were fine - and that's what counts.