How to replace logs from Vanilla

So, i was making a new Birch Forest mod, so i wanted to change birch logs with my new "Birchy Logs", and i checked in the forums and saw some old forums about it and nothing seems to work and, i DON'T want to change textures or models, just change the birch logs with my new "Birchy Logs", or at least change the bounding box of the birch logs