Subject Preparing for upgrade from 1.56 to 3 or 4

I have a few older FB databases that will need upgrading to later versions.  They are pretty large, and most have between 500-1,000 stored procedures.  Some contain many tables with millions of rows of data.  This is not likely to be a small undertaking.  Although I realize there are services out there that I can pay to do this, I'd like to do it myself simply because I have to manage these databases going forward.

Is there any documentation regarding doing upgrades like this available for review?  This is all on Linux and I realize that this process will take some time, so I may need interim support for DBs on upgraded Linux OS versions as I progress through this, but I'd like to be able to plan out the migration.

If anyone can point me to any slides, papers, presentations, etc. that might help with this, it would be greatly appreciated.