When you say replace do you mean literally hang an RRE2/U board downstream from an EP1502 or EP2500 or do you mean such that their usage of Mercury hardware would be lesser? I have an RRE2/U and an MR52 here, and while there are superficial similarities, unless they actually talk the same RS485 protocol (which as far as I know Merc uses an encrypted serial protocol) I don't know what the argument is about. There must be more than meets the eye to this.
Lenel was selling LNL-1320U reader interfaces as a direct replacement for the mercury made LNL-1320 (MR52). (Lenel still sells the board for use with their NGP panels (NGP-1320U, i think)