The 'preserved right to buy' is not an automatic right for those tenants whose landlord transfer to an RSL. However, recognising that most secure tenants won't vote for a transfer which removes this right most RSLs offer a contractual right to purchase the property on the same terms as if the Right to Buy existed for assured tenancies.
Given this it will be dependent on ter terms being offered by the new RSL landlord. If you haven't transferred yet then ask the question and make sure it is clear in the legally binding Offer Document on which you'll be asked to vote.
If the transfer has already taken place you may need to refer back to the Offer Document and, if it isn't clear, take some advice. My view, without having seen anything, is that you'll have transferred with the same rights as if you'd remained a secure tenant and that you'll still be able to purchase on the preserved terms of the RTB after the 5 years is up.