I would definitely take them up on their offer. Not only is the charging approach different with the OPN S 1-R than with Z Power (S 1-R charges via induction; ZPower via direct contact; S 1-R uses Lithium Ion, etc.) but the technology behind the S 1 platform is also an upgrade from your current OPN 1 devices (feedback canceling is much improved). Additionally, your warranty starts anew on the day you take the new devices, and the charging system is covered for the same time period (three years) as the hearing aids themselves (including the lithium ion battery). ZPower (at least with Oticon) is a one year warranty vs. the longer warranty on the hearing aids.
You mention being concerned about the weight of the devices. Having handled both OPN 1 and OPN S 1-R, I don’t think in actual experience you would notice much difference.
Chris