I don't seem to be having much luck using copy-paste in creative to move blocks between an HV and SV (even though I remove any HV specific blocks, most work with both ship types) but it just does not work for me. Especially now that there are turrets available on SV's I have seen many HV designs in the game/workshop that would be good candidates for SV versions, but there has to be a better way than just beginning with the correct starter block and duplicating the entire ship block by painstaking block? I am assuming that since every block of the original is "attached" to the starter block type, that it is permanent and not modifiable?
Save a new copy of the blueprint you wish to convert. Type the following command into the console, including the single quotes: prefabinfo 'Name of Blueprint' -convert SV This will change a Blueprint into a SV. Then spawn a new copy of the Blueprint which will now be a SV. Remove any HV specific devices such as the constructor, hover engines, thrusters etc. Resave when done. Use this at your own risk of course. Always keep backups.