What successful conversion/rewrite of software have you done?

I'm going for "most abstruse" here:

• Ported an 8080 simulator written in FORTRAN 77 from a DECSystem-10 running TOPS-10 to an IBM 4381 mainframe running VM/CMS.

I rewrote 20,000 lines of Perl to use "use strict" in every file. I had to add "my" everywhere it was needed and I had to fix the bugs that were uncovered during the process.

The biggest thing I learned from doing this is, "It always takes longer than you think."

I had to get it done all at once overnight so that the other coders would not be writing new, unfixed code at the same time. I thought it would go quickly, but it didn't, and I was still hacking on it at 6 AM the next morning.

I did get it complete and checked in before everyone else started work though!