Basic economy tickets can't be changed once the risk-free 24-hour cancellation period expires. If you did take a BE ticket and convert it to a main cabin ticket within the period, it seems likely that would be coded as an additional collection and be eligible for the credit if someone wants to try it. But that would only be $35+ at a time depending I think on domestic vs international and maybe the number of segments? Assuming add/collect remains a viable path then I think GC+remainder would be more efficient. Would be interesting to have the data point though.