It may be low on refrigerant but that's not a reason to replace it.
How old is it? Did it just start heating less?
Check the Water flow through the condenser, blockage of intake, algae in the coil, pump, etc. Check the temp of the water in and out. should be 10-15 colder in heat, and 10-15 warmer in. cool. Also, the coil with the fan should have about the same temp differences.
If you have the unit information, we can possibly find a temp/pressure chart unique to your unit and we wouldn't have use "rule of thumb" parameters. 410A or R22?
If you put it in the cool cycle the ice should melt. You'll also want to make sure the H/X isn't split from the ice too.