I would be a little worried about using heat on a sight - be careful! Don't try to put to much force on it, the inner split sleeve all the parts fit on is very thin & I have seen many examples with it broken or bent.
I think I have some old pictures of some broken ones I will try to find, it might help for you to see what the inside looks like. With the bottom locking collar turned all the way down the upper adjusting collar will have a little bit of up & down play, IF IT WASN'T BOTTOMED OUT, so you might be able to work it up & down to loosen it if it's frozen to the inner sleeve, if it moves a little up & down then it is only frozen to the threads of the stem.
added - Found the picture -