Short answer to the original query is, NO. RP must be installed as a clean install, there is no upgrade path available. Back-up all data to be ready for the transition. Dual-boot scenarios, where there is, also, a separate Data Drive are the most ideal. Have everything on the Win8 CP backed-up on the Data Drive. Install the RP & pull everything back, again, from the Data Drive. Software/programs will have to be reloaded.
Running a Beta OS as one's only OS & on one's only computer is not what ought to be done. If, for no other reason (& there, certainly are others) Betas expire. But, also, cus, cannot use upgrade paths.
Cheers,
Drew