Previous interests in a subfield which is different from your perspective advisor's
It is not at all uncommon (especially at BSc level*) for people to go into vastly different areas; their knowledge and skills in one area can often add to their abilities in another.

For simplicity in what follows, let your original field be A and your newly taught field be B.

Things can get a little hairy if you are determined to stay in A for your project (or, at least, it might not be as successful as having a supervisor with strong knowledge within A). If you are willing to make the transfer to B, and if you are willing to accept that further academia in A may be rather impeded (again, compared with originally taking a project in A), I do not see any issue with such a switch at this point.

There might be some circumstances in which "belonging to the same parent field" does not mean that A and B are even remotely related in their approaches and terminology in the modern day, and catching-up might be a lot of hard work. It is, I suppose, up to you and your friend to figure this out with available literature.

I am probably not giving you any information above what you already know - but I had to say the above just in case. What I would really like to say is the following: If you want to follow a path, follow it. Prior knowledge adds something to the group - not only will you have some unexpected tricks up your sleeve, but you may become the "go-to guy" for certain problems, and their knowledge will provide you with yet another set of tricks. Such mixing rarely comes to a detriment of either side of the table, provided you are willing to learn and they are willing to stretch you.

But if you are unsure on a path, do not take it simply because you dont feel you have a choice. I personally fell into that trap - in my case, joining a doctoral training centre in a much more general field than my own (Theoretical Physics -> Nanoscience). It turned out to be so general that most of the material was hand-wavy and useless. Now I'm back doing what I love, but it was half a year of plain old misery for me.