First of all, grab entdump - http://r-1.ch/entdump.exe
. Then extract q2dm3.bsp somewhere (eg c:\temp) along with entdump, then goto a cmd prompt (Start, run, cmd), cd c:\temp, entdump q2dm3.bsp > q2dm3.txt, exit
You will now have the original q2dm3 entities in the q2dm3.txt file. Open it in a good text editor (not Notepad) and search for 'hyperblaster'. You should find the entity declaration. Simply replace weapon_hyperblaster with weapon_rocketlauncher and save.
Now goto http://r-1.ch/r1q2gen.php
and for the map name, specify "maps/q2dm3.bsp", leave the checksum empty, then paste the contents of q2dm3.txt into the entity string box. Mash the button, and you will now have an override file you can use, eg as q2dm3-rocket.bsp.override which when started with "map q2dm3-rocket" or similar, will load "maps/q2dm3.bsp" with the modified entity string.