First of all, you could create a GEDCOM of your data in PAF2.31 and IMPORT that into PAF5.
The second way would be to install PAF 4. It will convert your data from PAF 2.31 for its use, without changing your existing PAF 2.31 data. You can then have PAF 5 convert this new data in PAF 4 for use in PAF 5. Again your data in PAF 4 will remain.
The major thing to be aware of when moving from PAF 2.31 to the newer programs is how you have entered your Notes in PAF 2.31. If you have strictly followed the guidelines recommended in the book by the Silicon Valley PAF Users Group, you have the option of converting those type of Notes into the new Sources section in PAF 4 & 5. If you have used this method you will know what I am writing of here. If you have not used that method, I would just use the GEDCOM method, rather than install PAF 4.
The other big change you will come across is that LOCATIONS can now be entered in full. You will probably have many long names abbreviated, or used state codes to save space. Any of these can now be expanded. When you have your new program up and running and data installed, read the HELP file and you will find all the assistance you need in there.