I assume you must have hacked the core of the 0.8.3 install because upgrading it to 0.9 should be fairly easy otherwise.
No other attachment plugins exist, this one was creating after a great deal of requests. They are not very easy to create.
0.8.3 is probably missing a few critical hooks my plugin needs to work.
You'd probably be better off going through 0.8.3 and turning your hacks into plugins and upgrade from there. Another trick is to convert your modified 0.8.3 into a SVN install and then using the SVN to upgrade to 0.9 . This is done by first downloading 0.8.3 via the SVN, then copying over your install on top of the SVN, then using the SVN to upgrade to 0.9
Keep in mind you should do completely backups including mysql if you try the SVN upgrade I mentioned as it might not be reversable via the SVN
0.9 has some speedups over 0.8.3 too