This topic, replacing or extending steerers, comes up every so often here. I suspect there's a lot of posts for the searching.

Mark's suggestion of only replacing the upper section is a good one. With threadless there's no issue of quill stem insertion and steerer IDs. So an internal sleeve (if you braze and not weld) would be a fine method to splice the two parts together.

Were you to choose to remove the steerer completely I would not use heat to do this. The fork is most likely brassed/bronzed together and the remelt heat level is a bit higher than the original joining needed and this level needs to be held over the complete joint, for as long as it takes to pull the steerer out. Think of a couple of flames or a really big rosebud tip and gas supply. If there's any pins (nails) that were used to hold the assembly together while joining these will need to be found and drilled out first.

I've recycled a couple of crowns when i was younger and even with a silvered steerer it takes a bunch of heat and time. Andy