If you are going to remove the old fixture anyway then unfasten it from the wall and see where the feed comes from. If it comes from the basement and there is enough slack then you may be able to re-use it, if it comes through the attic there may be so much adhesion to the insulation that it may be worthless, but the access to that wall cavity will prove valuable, both in fishing down to the basement and in "hand to hand" wire manipulation from the old location to the new one.

Usually spray foam insulation is installed to the inside of the roof deck not tot he top of the ceiling.

If you go the route of running through the stud, bore a hole rather than notching it.

Note that even if you remove the existing fixture, unless there's a lot of extra wire in that fixture, you're going to be making a junction there and you'll need to leave that box accessible (blank plate or whatever).

My approach is to plan as much as is possible the path in which the cable will be run before cutting any holes. A D'versibit might work well to drill a hole from your planned box location to the basement.
Planning would include looking at the situation in the attic and locating the place in the basement overhead that is directly under the planned box location. The absence of such concepts suggests that this job should be contracted to a professional.

Here is a link that might be useful: D'versibit