This was a great app, and sadly I am working on taking it out of one of my orgs now. It was truly a remarkable app for it's time and despite the clumsy UI, it was a great app. Of course you can now do all of this in Flow, but this app has worked for 9 years now without skipping a beat.
Now use Flow, Quick Actions or maybe try AppShark Lead Conversion for a custom Lead Conversion. Or of course use triggers.
I have written extensively about this product
Tips and Tricks https://docs.google.com/document/pub?id=1vC_r0S1SZZEfzgmIxzouZ0M7U9YfLU7RjT5EiDdmWoU
Blog Post http://thedetaildept.com.au/2012/03/25/three-great-free-apps-for-salesforce-admins/
Another Blog Post http://thedetaildept.com.au/2012/12/17/q-and-a-salesforce-custom-button-to-replace-convert-lead-button/