I know this is possible using a registry tweak but I've been unable to find it again. Anyone know it? Using this for an asus eeepc 1005ha
This depends entirely on your (built-in) sound card.
The microphone and headphone jacks are hardwired to the sound card. If and only if the manufacturer allows switching somehow, it's possible. If not, there's nothing the OS can do about it.
For example, Realtek calls this technique connector retasking. If your sound card supports it, you can retask your connectors with the Realtek Audio Manager. If applicable, this software should accompany your sound card / motherboard.
For other brands, consult your sound card's / motherboard's manual.
Yes it is possible to use a mic jack as a headphone jack and you don't necessarily have to mess with the registry.
I have a Asus motherboard and used Realtek HD Audio manager to change it. Turns out you can click the pink mic circle inside Realtek HD Audio manager and reconfigure it to headphones. Oh and this only worked for my front mic jack not the rear one.