You don't need a FileMaker V8 - just get a trial version of FileMaker V11 to convert the .fp5 files to .fp7 (FileMaker 7...11 are using the .fp7 format).
There is some work to do to maintain a perfect version under .fmp12 - but this is not depending to version 8 or 11..
You might have to call FileMaker support to get a trial version 11 since it's not on their trial-download-site
You can find older trial versions by searching the knowledgebase.
First, you use FileMaker 11 to convert to .fp7 format, then you use Filemaker 12 to convert to .fmp12 format.
I recommend that you read these two Knowledge Base articles on converting your files:
Converting to .fp7 format: http://help.filemaker.com/app/answers/detail/a_id/750/kw/converting
Converting to .fmp12 format: http://help.filemaker.com/app/answers/detail/a_id/10571/kw/converting
Be prepared to invest some time into testing and updating the converted files as there is a good chance that your converted files may not function perfectly. The KB articles cover many, but not all of the possible issues. Feel free to post back here with any issues you encounter that don't seem to have been covered by the KB articles.