I would have serious concerns "winging it" with R407c considering the glide when using a DX chiller vessel working off microprocesser staturated pressure/temperature properties of another refrigerant for the chillers safty protection.
I guess you could fit an external low pressure switch for protection but the chillers reliable operation would be compromised.
I've seen R407c and R22 use the same thermal expansion valves in some cases.
I've seen it a lot on split systems.
The bigger problem on the rtwa will be oil separation capacity. POE oils blend with refrigerant a lot better then mineral oils, which is why no traps are needed on high temp split systems using poe oils.
Carrier just sent out some info on converting 22 to 407' on a 100 ton machine it's like 17k to convert it. I haven't seen the labor involved . I'm shure if trane would let you convert it would be in the same relm , may want to replace the machines. Don't think it's worth the cost to convert. I don't remember what they said about capacity. Dozed off during that meeting