Unfortunately, it seems as they currently have no plans to do this.
Disappointment for 2016, 2017 MacBook Pro Owners
MacRumors got in touch with an Apple representative to ask about the new keyboard being used as a replacement for older models. Apple denied this as a possibility.
“When asked if Apple Stores and Apple Authorized Service Providers will be permitted to replace second-generation keyboards on 2016 and 2017 MacBook Pro models with the new third-generation keyboards, if necessary, Apple said, no, the third-generation keyboards are exclusive to the 2018 MacBook Pro.”
-Joe Rossignol, MacRumors
It’s possible that Apple took what they learned from the third generation butterfly mechanism, and will apply it to repairs for the second generation keyboard, but we haven’t heard of this happening yet. Apple hasn’t announced it as a plan yet either.
The new keyboard may not be compatible with the old MacBook Pro models, as the entire “top case” of the MacBook Pro has to be replaced when the keyboard is replaced. This includes more than just the keyboard, and even includes a battery glued to the chassis. While something like a keyboard may have been a drop-in replacement in the past, Apple’s dedication to making devices thinner and less serviceable has made such an upgrade impossible. For now, anyway.
Source: Joe Rossignol, MacRumors