There are various tools to convert Postscript (or PDF) files into PDF/A format.  Most of them are either difficult to use or expensive (or both), but Ghostscript (http://www.ghostscript.com/) is free and seems to do as good a job as any of them.  There is a script in the "Documents and Files" section of this site that uses Ghostscript to convert Postscript to PDF/A. You will need to customize the script slightly for your local environment, look for the string "% Customize". You run the script with the command

     pdfa "some title" input.ps output.pdf

Ghostscript is supposed to be able to read PDF files too, so it should be able to convert a PDF to PDF/A format. However, this doesn't seem to work well. Often the resulting PDF is mangled in some way.