Another alternative approach would involve retrofit of more than one unit at a time during low-demand periods and avoiding any outage during high demand periods.
There are no other alternatives when it comes to dealing with the retrofitting issue. 
mnli hypothesis: There are no other alternatives when it comes to dealing with the retrofitting issue.  premise: Another alternative approach would involve retrofit of more than one unit at a time during low-demand periods and avoiding any outage during high demand periods.