Migrate an iBGP full mesh to route reflectors without an outage

AS 64500 runs six routers in an iBGP full mesh (15 sessions, all on loopbacks): internet edges R1 and R2, core routers CORE1 and CORE2, and site routers R5 and R6. Both ISPs send a default route; R1 sets local preference 200 so ISP1 is primary. Fourteen more sites arrive next year, which would mean 190 sessions in a full mesh. You are asked to move to route reflectors during normal change windows, with no customer impact.