From man sfdisk
Change only the single partition indicated. For example:
% sfdisk /dev/hdb -N5
will make the fifth partition on /dev/hdb bootable
(‘active’) and change nothing else. (Probably this fifth
partition is called /dev/hdb5, but you are free to call it
something else, like ‘/my_equipment/disks/2/5’ or so).
So, in your case, it would look like this...
sfdisk /dev/hda -N2