Yikes.. That looks like a pre bud! Wait to see if it gets kinda cauliflower looking. They usually form on the tip of a established branch..Its time for Simmons to set blooms and that could be what happened. Or it could be a pre blush forming some new shoots. Either way its exciting g to see it .
I see you realize that the 15 gallon pot is the ticket to healthier trees that’s the smartest way to keep trees allowing the root system to grow.. Anywayz in a few more days you will know whats UP.. If its a bloom , I would clip it to allow all the energy to go to the plant and not set a flower ,, I know its painful
and Im glad its not me who will cut it ! You can ask around to be sure my advice is best
OUCH!
PS Your plant looks great