Suppose a Jewish woman converts to Christianity. Her children find out about their Jewish ancestry and decide that Judaism is right for them. Do they have to go through a conversion process? Does it matter if the coversion to Christianity happened many generations ago, and the decendents want to be Jewish (suppose the line is strictly maternal all the way down to child).
I give an answer to a similar question at My great great grandmother was Jewish - am I? The main problem is how long ago this occurred and what evidence is being brought. Additionally, there are references to Conversos who underwent "conversion" to show that they had totally rejected the (invalid) attempt to convert to a different religion and were coming back fully and completely (once they had escaped from Spain).
I also point out that I knew people who had performed a conversion misafek (because of doubt), because they did not have full legal proof even though all the evidence available appeared to show that they were Jewish.
You would need to check with an expert rav as to whether your particular evidence is sufficient to not require a geirus misafek.
In any case, the showing of the commitment is still a good thing and emphasizes the return.