Currently got haligen shower downlighters 12volt transformer 35watt bulbs, can I change these to 12 volt LED bulbs to the same equivalance using the origenal transformer??

Unless the LEDS are rated for AC /DC they will not work since your Halogen transformer is AC and LEDS are DC. In addition, since LEDS draw such low current, the regular transformers don't induce properly. Typically a LED circuit used a special transformer that is then rectified to DC or the LED itself has a.built in circuit. Read the specs on the LED.