You can download e2resizeblk here.
This tool allows you to change the block size in ext2/ext3 filesystems keeping the data stored in them.
Without a tool such as this, you need to reformat your filesystem to modify the block size.
The tool is still in development, so if you use it, you will probably loose your data (note that it could happen even
if it had not any bug).
Feel free to send me an email for any comments or suggestions.