The copper cable is and enters the 2nd floor there is no cable ground level. The cable comes from a pole from the back of the house and then is attached at the top of the house then runs down about 3m and then enters the house to the bedroom.
If the new junction box has to go ground level they will just have to extended it back up the wall to go through the same hole or next to the copper cable on the 2nd floor as its not going in ground level thats for sure as that would be such a ball ach to get the network cable from there to the other side of the house
but if they enter the 2nd floor and place it on the inside wall i can Easly run a cat6 cable from there to the router. if bt cannot accommodate then it will not get installed simple as that.
It used to be the case that the CSP had to be at ground level even if it meant a cable going down to it and back up again. However, there have been a couple of rare occasions when the CSP has been located at a higher point. Not sure how the Openreach engineer managed to use a splicing machine at something other than ground level.
You will need to agree CSP, ONT and router locations with the engineer on the day.
Where the current copper service enters is totally irrelevant.
This is my FTTP install and you can see the ONT and the small cigar like box is my CSP. My fibre comes from pole and is secured at eves and then enters loft just below eves and then is installed in my loft conversion
My house is over 60 years old and when I switched to FTTP they left the master socket in place and installed the new box in the back room and that was it.