American standard,heat pump, 2&1/2 ton with emergency back up,(electric strips) outside temp is 25 degrees & have the thermostat set at 75 degrees unit runs constantly but air output is barely warm, even with emergency turned on. I have a 3 ton compressor with heat pump for a spare. Can I change out with out having to change the inside unitm and would this solve my problem?

WOE cowboy... Not so fast
Why not try to fix it first. You may have a refrigerant leak, or since it's that cold, your auxiliary heaters might not be at full capcity. Those are arranged in banks of kilowatts. From 5 to 20 kw depending on your system size. Call an experienced pro first. Still want to replace it? Sure... that ought to do it as well.

I agree with Covertaire I'd say either a refrigerant leak or the axillary heat isent running @ full capacity.

There is a diode on your electric heat strips. It is fkd up or it would heat or the electric heat strip is out. Just so you know heat pumps are for 32 plus weather. Get rid of it and go gas or all electric..