I have a 400 watt hps and reflector and want to know if I can take the actual reflector off and use on a 1000 watt ballast and light. Im speaking of placing the actual cord and socket into the 400 watt reflector. I guess my question is will the socket fit or are they different sizes.

If they are both mogul it should fit as long as thge reflector is big enough for the lamp. Worst case you might have to mod the mounting a little (use the McGyver spirit all stoners are blessed with).

socket should be the same. in fact,you should be able to disconect the cord from your old ballast & put it to the new ballest and swap lights.
if the new bulb fits the old socket, you are ok as long as you match the bulb to your ballast.
sockets come in 2 flavors medium and mogul, I think 250 watts is the cutoff- anything under is a meduim base and anything over is mogul( I think 250s come either way but not sure.

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Where are we going & what am I doing in a handbasket?

ok cool that will save about a hundred, my cord is connected to the 400 though so i cant do the simple swap wish i could though