Can I replace my halogen bulbs with led bulbs without buying new lamps?

LED bulbs CAN work in enclosed fixtures IF the equivalent wattage is the same or less than the fixture's wattage.

Example:

Fixture says not to use more than a 60W bulb.

You can use a 60W equivalent LED bulb in there if it fits.

Heat is heat, regardless of the type.

And LED's have internal ballasts to reduce the voltage and that causes heat.

Read the LED package for further information.

Also, some LED's are dimmable.

f the have the same base and voltage rating, yes. Dimming may or may not work however.

yes.  the LED bulbs are designed for exactly that.  Note:  some dimmers may not work with LEDs ... test before investing big $s in LEDs

Led bulbs won't work in enclosed light fixtures.

Any light fixture that allows free air circulation around the LED bulb will be fine

Yes the leds with the screw base will have it's own driver inside. Since you aren't using halogen bulbs anymore you would need to change the wiring in the lamp fixture to bypass the transformer. Make sure you know what voltage you are running and get the right bulb