Transition from Old to New SCADA Master

Hello all,
I am a radio guy working on a project where the integrator wants to bring on line their new polling computer but at the same time alternate back over to the old system letting it get updated and then back over to the new computer to test and slowly bring on the new sites. The problem apparently is that the old system uses Modbus Plus and the new uses Modbus RTU, apparently the device he thought would allow him to alternate between the two computers is not Modbus Plus compatible. Anyway, I'm just attempting to find something out here to see if anyone has come across this type of situation before and how you got over the hurdle. I'll most likely tell him about this site so he can discuss it with his fellow PLC guys. Thanks for any help you may be able to offer!
Chuck

A list of people who deal in protocol conversion:

www.digi.com
www.lantronix.com
http://www.prosoft-technology.com/
http://www.miille.com/
http://www.anybus.com/
http://www.hilscher.com/products_list_group.html?pg=22&upg=12
http://fieldserver.com/products/gateways
www.*********************
www.protocolconverter.com
http://www.equustek.com
http://sales.trianglemicroworks.com/ProductPage.aspx
The one asterisked out is
www dot data link gateways dot com (take out the spaces)

Is he sure there's not a BM85 or other Modbus Plus to Modbus RTU gateway before the data goes out onto the radio network ?

I've never seen Modbus Plus carried natively over radio. I'm not saying it's never done, but I've never seen it.

Thanks guys! (Dan and Ken) I called him to refer him to here but he said he was going to trash the idea and go a different route....what ever that is.
So, thank you for your replys and if anything else comes up, I'll have him post directly.
Take Care, Chuck