Re: Is this possible?
Yes, you certainly can.
take your time, and cut the old head tube away from the welds. Leave the welds on the frame tube.
Then carefully grind back the welds to virgin tube, while shaping the end of the tube to meet the new head tube. You want a very close fit to the radius of the new tube. The better this fits, the better the weld.
If you take it to a welder & it fits very close, the welder will be happy. Be sure to sand the paint back at least 2 inches from the joint. TIG welders like to see clean bare metal in the weld zone. Sand any rust from inside the frame tubes at the end as well.
Prep work is the most important part of welding. if you take dirty, poorly fitted parts to a weld shop, they will grab the MIG and butter them together because the arc will scour the crud & rust while filling the big gaps.