I'm pleased to see that the software that TConnolly linked to claims to support 'all models' of the PLC-2. There was an Australian product a while ago that supported only a handful, and it was really tough to figure out which ones.
In each PLC-2 system I've migrated, I had issues where modifications had been made that were not documented, so uploading the actual current program was crucial.
The other major consideration I've run into is that systems this old are often running critical processes that need to be re-engineered when the new controls are put in. One example was a PLC-2 running a burner control system in a power plant: the regulations have changed a lot in 30 years, and we had to do a whole set of very specific calculations and design changes in order to install a modern controller. A rung-by-rung conversion would have functioned identically, but it would not have satisfied the regulators.