What do you need to be a bicycle mechanic? Not much at all, bicycles are simple enough contraptions, and anyone with a hint of mechanical ability should have no trouble picking up the trade.
But, no one leaves being an auto mechanic to work on bicycles; normally the opposite occurs. As an entry-level bike mechanic, you'll probably spend a lot of time on your feet assembling low-end kids bikes, and eventually mid-level commuter bikes. It's not difficult work, and I myself enjoyed doing it; the only thing I didn't like about being a bike mechanic was the abysmally low pay, which is roughly fast-food restaurant level.
What is the current pay level for auto mechanics in your area? When I was a mechanic I earned a percentage of the hourly rate ($50 an hour at the time, of which $25 an hour was my share). As the hours for a certain job are at a fixed rate, if you could finish the job more quickly, you could do an extra job, or even two in the same allotted time. It was possible to make $75 an hour during busy weeks/months.
Bike mechanics earn an hourly rate, and a low one at that. Both of the shops I worked for were struggling financially, and I was often paid in bikes or parts (if at all). Not a bad deal when you are living at home with your parents, but not so good if you have your own bills to pay.