you'd better copy the contets to somewhere else and reformat it to fat32. you can use tools like partition magic to convert it but it will take a looooooooooong time if the partition is large and full, and to make it worse, using such tool is risky - you may lose the data, that's why ms does not offer such tool.
why do you want it to be fat32? if you do not have win9x system that needs to access the disk, there is not a single reason to use fat32.