is it possible to set a free play game into a different game type

If you mean single-player, yes. While at the main menu go into your game mode settings. Look for 'mods' and select the type you would like to play. From here you can toy around with different mods.

i tried that how ever when ever i type in a game mode like twinswords4 it returns me to classic

If you are using commands then type the following;

/set mod twinswords4.tbm

Make sure you include the '.tbm' after every mod.

Then type /reset which resets the mod.

Press: "control + m" then highlight what mod you want. click load.