Been a while since I posted anything... And I found this in my drafts. I can't remember if this is correct but no one is wrong on the internet so...

prostrct create db-name db-name.st -blocksize 8192

procopy $DLC/empty8.db db-name

While you can create a database with a new block size as above easily enough. You have to use DUMP and LOAD to convert from one block size to another.

http://knowledgebase.progress.com/articles/Article/P6430