In terms of CO2 specifically we can think of it as the 'embodied carbon' which the CO2 created during manufacturing of the parts, transportation, and assembly. Then we have the 'operational carbon' which is the day to day impact.
So to amortise the carbon cost we take the embodied carbon, plus the operation carbon over a time frame. If the operational carbon is negative we would expect that at some point it would break even.
So to help with the English, I don't think it makes sense to talk about the amortisation during the change over since that is just the embodied carbon. But I think it definitely makes sense to talk about the amortised cost over some other time period after the change over.
So to amortise the carbon cost we take the embodied carbon, plus the operation carbon over a time frame. If the operational carbon is negative we would expect that at some point it would break even.
So to help with the English, I don't think it makes sense to talk about the amortisation during the change over since that is just the embodied carbon. But I think it definitely makes sense to talk about the amortised cost over some other time period after the change over.