I assume your electrician checked, but make sure the lines feeding the subpanel are the right size to handle the added load of the heat pump. I don't claim to be an electrician, but the code book has pretty straightforward rules regarding voltage/amp load/wire gauge/distance. 90A on the existing one sounds big. You may have a 50A and 2 20A breakers in the subpanel but that does not mean the load is that high. Check the breaker at the main panel.