Both .pl1 and .pl2 files are used by 3D Home Architect. It is not clear whenever one can be simply converted another, however, you should be able to open both with 3D Home Architect and work with them. Some sort of copy and save is highly likely.
Convert 3D Home Architect room plan to 3D Home Architect second level floor plan file.
We found one software available in the database for this conversion.
Microsoft Windows software - convert pl1 to pl2 on Windows
Old house plan making program
Convert from pl1 file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.