Why do you want to switch to RO?
Are you having problems with your tap water in your tank?
It would probably be best not to use pure RO water in your tank, you would want to use a half and half mixture (half RO, half tap) for water changes as well.
If you aren't having any problems, it would be easier to stay with your tap water. Your fish are also used to your tap, so switching over should be a slow process.
EDIT:
What fish do you currently have in your tank?